During my joyrney on the Yesterweb, I noted some obscure sites that could be described as social media. They are as follow:
your friends' updates
status.cafe
https://www.dreamwidth.org/ (has some NSFW stuff, also claim to be about freedom)
Yarn.social is a decentralised self-hosted social media based on the Twtxt format that has a privacy-first focus. There are no ads, no tracking, and no personal information is ever collected or stored.
yarn.social
SpaceHey is a retro social network focused on privacy and customizability. It's a friendly place to have fun, meet friends, and be creative. Join for free!
spacehey.com
Aside maybe from Dreamwidth, I don't think I know anything that fit what you seek.
I have heard some people singing praise about XMPP. You might want to check it out. From what I understand, it's a decentralized self-hostable thing. Haven't had time try it myself yet.