"Lots" is the answer that makes the most sense for me - I write in a lot of fandoms, mostly TV series, I started out in Buffy fandom so I'm on a discussion board that's part of Twisting the Hellmouth, the big Buffy crossover site.
I'm also involved in literary SF fandom, though not so much on line - I go to conventions etc.
I'm also on two discussion boards for laboratory technology, one for BMW motorbikes, livejournal, etc. etc.
