There's actually a lot of software projects, Mastodon is just by far the most popular one. From top of my head, there are more extensive lists out there: Pixel

There's actually a lot of software projects, Mastodon is just by far the most popular one. From top of my head, there are more extensive lists out there: Pixelfed (Instagram alternative), Peertube (YouTube alternative), Pleroma (another Twitter-like microblog), Bookwyrm (Goodreads alternative), Funkwhale (like a mashup between Soundcloud and a podcast host), Owncast (Twitch alternative), Mobilizon (event organizing), lemmy (reddit alternative)... They're all rather tiny and unpolished, having even less resources to work with than Mastodon, but you should be able to follow a user on any of them from Mastodon. At least in theory, haven't tried with all of them. The annoying thing in my opinion is that you can't have a set-in-stone identity and then use different frontends for different purposes, kind of like you can on Facebook for groups/events/marketplace/stories. You have to have an account on all of them to make use of all features, even though they're all relying on the same protocol to a certain extent.