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?