Besides Lemmy which I love a lot, I’m also enjoying a microblog platform called Catodon, which is much less basic than Mastodon and has a very nice interface. Pixelfed is also fantastic if you like photography rather than just being Instagram influencer nonsense. Where else are you hanging out? What have you liked and not liked?
Matrix isn’t using Activitypub, so it isn’t compatible with the rest of the fediverse. However, with Matrix anybody can host a chat server and all servers interact with each other. So just like the rest of the fediverse, Matrix is fully decentralized, free, and open source.