[aaroneous]

I think the answer really depends on the niche you want to go after. Do XBox360 fans need another social network login/pass to remember? I'd assume not, a Facebook community probably would be easier for them.

But, I'd also assume that there are viable opportunities out there that, by the nature of the niche, would function better as an isolated specialty site. Things like a social network for elementary school children would make sense in that context.