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Been part of Harry Potter, the academic and philosophical side of it. The conventions are a whole other world, and filled with all the creativity and camaraderie that I've never gotten with "normal" people. Most all the people I know in it hold doctorates in everything from literature to hard sciences (several are rather famous in their fields and keep their HP fanning a secret).
Various anime fandoms, although I always end up in the intellectual/adult camps that shy away from infighting and focusing on characters' sex lives. We mostly wrote speculative fic based on the plot lines and minor details that could be important. I also cosplayed.
I would join the Sherlock fandom, but the few fans I met are strictly into the slash part, so I have little in common with them. The only person who I can discuss it with is C since he's not into slash and is actually interested in the writing of the show. Same situation with Doctor Who.