Instead of being run by a single company that builds the software and runs the servers (Facebook, Twitter, etc) Mastodon is more like email. You sign up for an account with some provider ("server"), but can talk to people regardless of which provider they're signed up with ("federation"). There's an open protocol (ActivityPub), open-source server software (Mastodon, Pleroma, etc), and many servers you could join.

Overall I'm pretty pessimistic about Mastodon, even if we only imagine futures in which lots of people move to it, because of spam. Handling spam on a centralized platform is hard but manageable; federation changes this dramatically for the worse. Imagine you're running a server, and you get an incoming message (comment, like, DM, etc) from an account on another server. Today the default is to let it through, but as Mastodon gets larger there will be more and more money to be made in spamming there, and the larger a fraction of those incoming messages would be spam. Many of the signals centralized platforms use to detect spammers (local activity, account age, IP addresses, etc) are not available in a federated context, leaving server admins (and the software they delegate to) very little information for identifying and blocking spam. It's at least as hard a problem as email spam filtering, probably harder because of shorter messages, and I expect this makes it very hard to run a server that doesn't drown its users in incoming spam and reliably gets its messages out to other servers. Maybe we get to the equivalent of everyone using Gmail/Hotmail/Yahoo and anyone with a different provider has a very frustrating time?

Still, I'm happy to give it a try, and if it does collapse in a pile of spam, oh well. Does anyone have a server they'd recommend? I'm not that excited about the idea of joining a server at random because I'm trusting the server admin not to impersonate me, and also because apparently being on a server with an interesting local timeline is fun.

(I considered self-hosting, but an email server is already more complex than I want to manage and a Mastodon server is much more so.)