Long ago there was this thing called foaf https://en.wikipedia.org/wiki/FOAF and also https://en.wikipedia.org/wiki/Pingback ... it was the closest I've seen to completely decentralised social media.

Webmention is the modern counterpart: https://indieweb.org/Webmention

(The IndieWeb wiki is probably the best resource for exploring the personal website-based social networking tech nowadays. I recommend the author check it out and maybe iterate on that instead :)

Is any of this stuff sort of out there working and I maybe used it without realising?

Or it's all super niche for "personal website-based social networking" enthusiasts and never took off, because big players didn't implement it and we need them to, or whatever?