It's amazing how you can wind up on a subreddit and realise that you're nowhere near the bottom of the rabbit hole that you've been calling a hobby for the past n years.

I don't know if this qualifies for what you're after, though, because these communities are:

* based on users actively seeking out forums based on what they consider themselves to be interested in, and

* each community is centred around a single topic. There might be a subreddit for people who love Fraggles and a subreddit for people who love pop-tarts, but no subreddit for people who love both Fraggles and pop-tarts.