With loads of folk making the move from Twitter to Mastodon, a similar-ish service which is free and open source, there is lots of talk of the fediverse; a set of federated services built by developers, and running on thousands of volunteer run servers.
Here's a list of some of the best fediverse services:
Diaspora The original Fediverse distributed social network.
Friendica lets anyone or any organisation literally create their own private "facebook" on the fediverse and make it accessible and federated with the rest of the World.
Hubzilla Friendica improved
Peertube Youtube competitor
Pixelfed Instagram-ish
Lemmy Reddit-ish
Funkwhale Soundcloud/bandcamp-ish
BookWyrm Goodreads
Castopod Next generation podcasting software