Coalescent

I think the serious point is that I would never tell someone they're a member of fandom just because they read sf. I think that has to be a self-identification, because most of the time being in fandom isn't actually about sf, it's about being in fandom. On the other hand, if you talk about a community, you're talking about the social group that thinks of itself as fandom and all the other people as well, and that's a useful thing to be able to do.