I used to belong to mailing lists and a forum, both in the HP fandom, but I stopped participating when I discovered LJ because the format suited me more. Besides, my most common "activity" in any fandom is to read fics. The mailing lists were author-owned, or specific fanfic oriented, so there wasn't much of a difference between those and LJ. The forums were more general and RPG centered, and I'm not really into RPG. I did join another forum later, in the CSI fandom, planned to check it out regularly, but then I just couldn't be bothered and I'm no longer in that fandom, so it's doubtful I'll ever go back. At both of those forums, some members were slashers and the rest was fairly tolerant to both slash and fanfiction writing, so I didn't have a bad experience in that regard.