It depends. You could make it work if you gathered enough momentum and offered good resources or other goodies that couldn't be found elsewhere, even if it was just a very organized list of Tumblr posts per fandom. (That's actually an idea, we could at least start Fandom posting by archiving the zillions of master lists scattered all over Tumblr). You'd need a dedicated mod or mods, and those mods would probably have to be unanon. Lead by example and all.

On top of that, you'd need to get people posting, which means advertising, cajoling your friends, and providing things to post about. I don't know if generic 'fandom discussion' would cut it. So, that's the thing: I think it could work, but only with a lot of effort and solid modship.