This is something I've been wondering about too! Social media is barely "social" but a lot of the private circles/Discord servers/etc. were things I stumbled into by accident or had to be invited to. But the issue is before social media existed, I had plenty to do online, but now there's not much else besides social media... There's not nearly as many active forums for any little thing, so many end up dying after a short time. Even the forums I did use to frequent as a teenager like Pokemon fansites moved to more active Discord servers and have a completely different culture now.
I have a Dreamwidth account I've debated on dusting off, but finding what communities or journals are active is hard to figure out too. Some personal websites have links to their own servers and IRC channels, I've considered downloading IRC just to check them out.