Twitter can actually be good for this. Engage official accounts and popular people in your fandoms. I became a prolific Twitter user only after I tried to meet up with other people who loved Zero Escape. Just follow people and you slowly build up a circle over time. It takes a long while though. I'm at like 320 followers after a couple years. But that's fine.
Connect on Twitter with people you know from elsewhere so that you can interact with THEIR circle too. Same goes for other networks.
What really, really, really helps is finding small communities related to your hobbies. Just search the name of an anime, or game, or book, etc. on Facebook and the like- most pieces of media have many little fandom groups attached to them (not just on Facebook- subreddits are one example), and those are usually pretty inclusive so you can find a place for yourself in there. For example\ I joined a fan group for a not very popular young adult novel series seven years ago, and a majority of us really stuck together- we're very tightly knit and I'd count them among my closest of friends= they are actually the only reason I still use Facebook with any regularity.