Imagine if you needed a Gmail account to send a message to a Gmail email address. Or you needed a Yahoo account to send a message to a Yahoo email address. That would seem completely absurd. But that’s exactly how Twitter, Facebook, Instagram, YouTube, and other social networks work.
By contrast, the fediverse uses ActivityPub to communicate across instances. If you have an account on Mastodon.social, you can still interact with someone on Indieweb.social because they utilize the same protocol. And everything just works — you can follow, reply, comment, and interact with other users across instances in every way you’d expect to.
In terms of the software itself, there’s Mastodon, Pixelfed, PeerTube, and more. And it’s all free and open source. So not only can you sign up to join an instance that someone else maintains, you can also setup and maintain your own server.
Fediverse.Party offers a pretty good overview of the most popular software in the fediverse. And there’s plenty of information about user statistics, links to tutorials, lists of instances to join, and more. I’ve been toying around with Pixelfed recently and have been very impressed. I think this technology is going places.