There are fandoms that are a threat, and fandoms that aren’t. When someone (namely, Seema) gets interested in X-Men or JAG fandom, I don’t worry. On the other hand, when someone (namely Jerie) gets into SG-1, I know I’ll never see that fan again.
Least threatening of all are the TV dramas and comedies like JAG or Friends. West Wing was the biggest thing there, and it seems to have come and gone. The appeal of real life is clearly dwarfed by the appeal of…dwarves. For me, a show needs the element of the surreal to hold my interest, and that seems to hold across fandom.
Fandoms based on a couple of movies combined with books or comic books (Harry Potter, LotR, X-Men), while huge, don’t scare me. I don’t see myself being tempted by that kind of fixed canon, and so I don’t worry that my favorite writers will be lured away. Maybe they will be anyway, but how long can it last? The difference between a fix every week and a fix every year is too great. Note that there is no second tier of fandoms here - other movie or book-based fandoms are either too to notice or short-lived fads.
What pushes a fandom over the edge into greatness? I think it takes a long-running TV series (or series of series) that’s about science fiction or the fantastic. So the big fandoms are Star Trek, X-Files and Buffy, with Stargate and Smallville the new contenders. They have both allure and staying power.
I’m even afraid I’ll end up in SG-1 fandom.