Updated: 20221105
After some discussion, it was mentioned that I should probably update this document in light of recent events and to clarify certain things that are somewhat confusing. Such is the danger of writing about something you know really well; there are parts that you forget to explain, jargon that needs to be unpacked, and suchlike. This is all stuff that an editor is supposed to call you on. But, I don't have one, so here goes.
If you've been kicking around on the Net for the past year or so, you've probably come across a thinkpiece or two about Mastodon, an open source social network that's kind of like Twitter, kind of like Facebook, and kind of like... well, nobody's really sure what else would fit there. It's a bit of a wildcard. That seems to throw a lot of people, and because this is the Internet we're talking about that means a lot of "this could never possibly work" posts, nevermind a busy network of several thousand instances and several hundred thousand users doing everything from venting their spleens to asking for (and surprisingly oftentimes receiving) assistance, collaborating on projects, goofing around, and mourning their fallen...
It's hard to wrap your head around. Let me see if I can help a little.
Unless you never use it Twitter these days is a cesspool of flame wars, hit-and-run shitposting, shoutybots, nazis, propaganda botnets, and people generally not having a good time. It's gotten a lot worse since Elon Musk bought Twitter and immediately set about making it worse. Facebook went full cyberpunk a few years ago and you're fortunate if you ever see posts from people you actually live with. Tumblr recently committed brand and business seppuku by hamhandedly hunting down and removing anything that even vaguely smacks of adult content (but you'll have no trouble finding nazis or their fellow travelers). These things have lead to a large number of people getting fed up with social networks but they still want to find people to, well, socially network with.
The Fediverse is a network of servers that communicate with each other to form a vast social network. While the history is more complex than I tell it, for our purposes it started with Mastodon, open source microblogging software that resembles Twitter in some ways. First there was mastodon.social, the instance run by the creator of the software as a proof of concept. Then more and more people started setting up their own instances - thousands of them. Some instances are general in nature and you can find folks of just about every kind. Some instances have a particular theme, like cyberpunk, LGBTQ culture, the furry fandom, Star Trek, Homestuck, or just about anything else. There is even an instance dedicated to running bots (botsin.space).
The question has been asked by a couple of folks, if you're on a themed instance (let's say, information security), can you only talk about that stuff there? Do you have to create an account elsewhere to talk about other things?
The answer is no, you don't. By and large, themed instances are geared for folks of a certain sort while not proscribing things. If you want to talk about your brand new fursuit on a site for academics (which you joined because, in your mundane life you're a college professor, for the sake of discussion), the mods won't yell at you for doing so. Instance themes are for general sorts of people, not prescriptive of content. People are multifaceted. If you want to do that, go right ahead. But you don't have to.
Another commonly asked question is, do I need an account on every instance I have friends on? The answer is no, you don't. Federation means that servers talk to each other so you don't have to be on the same server. Take e-mail, for example. (Yes, I'm showing my age. Go with it.) If my e-mail address is at gmail.com, and your e-mail address is at example.com, we can still send mail to each other, right? You send me a message, example.com throws it over to Gmail, Gmail drops it into my inbox. The Fediverse works the same way. I'm at hackers.town, you're at illuminati.gov, and we follow each other. So, when I post you will see it, and when you post I'll see it.
A third question is, can't the server owner see my private messages? The answer is yes, they can if they care enough to do so because it would involve a great deal of effort and possibly writing custom software. However, the same thing can and does happen at Twitter or Facebook. As Mudge stated when testifying before Congress in 2022 it's pretty common at Twitter due to the generally poor state of internal security. So, if there is anything especially sensitive or illegal that you want to talk about, any social network (by definition) is the wrong place to do it.
There are so many instances out there that there are a number of sites which help you figure out what instance you might want to join. You could start at the Join Mastodon website, or perhaps instances.social in the form of a brief quiz. More recently Mastodon Servers appeared to help you find your way.
Some instances are invite-only. This is done for any number of reasons, from slowing a flood of signups to requiring that an existing member of the instance vouch for you. However, there's nothing that says that you can't just e-mail a server's admins and ask for an invite. By and large folks are pretty polite, and if the answer is "no" they might help you find someplace where you'll feel at home if you ask nicely. Remember, the denizens of the Fediverse are people like you and me.
This brings me along to something I find both interesting and appealing about the Fediverse: Unlike other social networks you don't see a whole lot of brand-building happening. That's not what we're about. What you will see is a lot of people who're being remarkably genuine about themselves and their lives. None of us are perfect. As my grandmother used to say, "That's why you've got an eraser on your pencil" (and, I would add, a backspace key on your keyboard). When people reach out for help, they seem to get it more often than anywhere else I've seen in a long while.
You're probably wondering what makes the Fediverse different from other social networks. What's the secret sauce? For starters, the software that makes up the Fediverse has content moderation features built in. If people start doing uncool things like posting some generic horrifying thing without warning anyone first the moderators will smack it down. If there are folks on other instances that are giving you grief you can block them. You can filter out keywords or hashtags that you don't want to see on your timeline. If an instance becomes a real problem or does something patently uncool, folks let their admins know (if the admins don't know already because Fediverse admins tend to keep in touch with one another) and they'll defederate with it; in other words, they drop the banhammer on that entire server. Ultimately, we curate what we see and there's no reason to put up with bullshit if we don't have to.
In the Fediverse there are things called CW's - content warnings. In essence, you can hide the content of a post so that you'll only see it if you click through. You'll see this done for spoilers, things that may not be of wide interest, people venting but letting folks opt in if they choose, politics (many of us came to Mastodon because we want to get away from all-politics-all-the-time), to hide the punchlines of jokes, or just because they feel like it. As a general rule, ask yourself "If I had a really rough day, would I want to see this thing or would I want to click through?" and CW accordingly. Additionally, Mastodon allows you to upload multimedia content, including images and brief videos, which can also be flagged as sensitive content so that they won't be visible by default. This does not necessarily mean that the content is not safe for work or otherwise objectionable, some people do so because they want people to decide for themselves what they want to be exposed to or to avoid causing problems for people with limited bandwidth.
Additionally, multimedia is federated along with posts so you don't have to be on the same server for that, either. It is considered polite to include a brief description for the benefit of folks who use assistive technologies, like screen readers.
Of course we also have emoji. Emoji are so well supported that you can practically have a conversation with them. Every instance also supports custom emoji, which are generally referred to as emojo. I think it was a misspelling that caught on. At any rate, emojo are federated so if someone uses one local to their instance it rides along on their post so that everybody sees it. It's been my experience that instance admins are happy to add new emojo if you're polite about asking.
If you're new to the Fediverse it is considered polite to post a brief introduction using the hashtag #introduction. It's a good way of meeting people. Some people pin their introductions to their profiles so they're easy to find. We also have the quasi-tradition of #ff (Follow Fridays), where some people post links to some other people they enjoy following. There are also several collections of lists of users who've opted in to help new users find interesting to people to follow. The first is called Trunk, and the second fedi.directory (which is organized by category). There are also entire groups of people called guppe (which seems to be a pun on the word 'group') that you can follow all at once. It's a little hard to describe, so here's an example:
Let's say your thing is biking. You fix and recondition bicycles, maybe you're a semi-pro bike racer, maybe you do distance rides for fun. Let's also say that you want to meet other folks into bikes in the Fediverse. So, you follow the guppe called fedibikes. When you do, you will see everything everybody in that guppe posts, and you can interact with them as usual. If they're too high traffic or you found only a couple of folks that you want to hang with, you can follow those individual folks and unfollow the 'fedibikes' guppe.
Every once in a while someone might want to switch instances. It happens. The software comprising the Fediverse allows you to export all of your data - all of your posts, a list of everyone you follow, your filters, and your blocklists. You can then set up an account on a different server, re-import that data, and pick up where you left off without having to start over from scratch. Because it is possible to move to a new instance whenever you want, it is considered good form to leave a forwarding address of sorts on your old account so that people know where you've moved to. This notice will appear at the very top of your old profile. As you can see, I've done this once before. I don't have any problem with mastodon.social but the essence of federation is that people can move elsewhere in the network if they so choose. Decentralization is meaningless if you don't actually do anything with it.
A user on a Mastodon instance has three timelines, or streams of posts visible. There's your personal timeline, comprised of posts from everybody you follow. Whatever server you're on has a local timeline comprised of all of the posts from everybody on your instance. Finally, there is the federated timeline, which is every post from everywhere else across the Fediverse. Skimming the latter two timelines one in a while, and maybe searching for a hashtag or a couple of keywords once in a while is a good way of finding people to follow. You don't have to follow everybody under the sun. There's no rush. Take your time and find the folks you want to hang with. Do it because you want to enjoy where you are and have interesting people to talk to.
Let's not forget the mobile apps, because everything's an app these days. Mastodon has quite a few for Android and iOS, and most of them are free.
It's been said that if you're not paying for the service, you're the product. Time and time again this has proven to be true. Facebook, Twitter, and so forth make their money selling information about their users to stay in business. The Fediverse isn't like that. Server admins spend their own money to keep things running. Smaller instances tend to eat the cost as if it were a hobby. Bigger instances fund operations with Patreon, Liberapay, or ko-fi to help defray operating costs. Others hold periodic fundraisers to cover the cost of hosting or hardware.
Addendum: I collect links to fediverse-related stuff in my bookmark collection, so if you're looking for more information you might find something useful in there.