I think for me being in the fandom of a show, as opposed to being a fan, is when I've gotten involved with the other people watching the show, apart from my interest in the show itself. For instance, for me, writing and sharing fanfic is being a fan, and arguing very seriously with people about whether it's canon and whether my characterization is wrong is being in the fandom. When I was hanging out on the OB and started wrestling with some folks over differences in opinion about Inara's characterization, I realized I was getting more into the fandom than I was comfortable with. It stops being fun when it becomes Very Serious and Important.

Wow. Have I been a part of a kerfluffle?

If so, it's kind of tiny. Maybe just a fuffle?

A kerf?

It stops being fun when it becomes Very Serious and Important.

And for some of us, that's where the fun starts.

Seriously, I'm intrigued by your implication that being in (a) fandom is something negative, as opposed to being a fan of the thing -- and also by suela's very high bar for self-inclusion; not in any "I'm right and you suck" way in either case, but because the whole definition of fandom is so different from mine. I use the word almost exclusively in the broad and multifandom sense -- fandom at large, as a culture, a group, a set of activities, maybe even an identity. I've gone long stretches in the last few years when I haven't had an active text that I'm even bothering to follow, let alone that I've written or argued about, but I've never been out of fandom; that's more a way of seeing the world than anything else.

Which isn't to say that there aren't fandoms for particular texts; I'm not at all into some of them, bitter about others, newly in love with one or two, and I can certainly use and recognize the term as an identifier: but I'm not in HP fandom is the same as I'm not that into HP. And it's never my first definition of the word "fandom".

eta: I vote "fuff"

Well, I view every single person here (including myself) as part of that fandom. Of course, people here can generally spell, however.

I wouldn't agree that posting on a bulletin board about anything makes one part of fandom.

I vote "fuff"

Second.

I consider myself footloose and fandom free.

My last fandom was NSYNC. Before that it was BtVS and Angel. Before that I was without a fandom.

Huh. I burned out real fast on the whole fandom thing, didn't I?

I know people who consider fandom their fandom.

Hmmmm. I have no flag in any particular fandom, anymore. I am fandom peripheral.