That's an interesting idea. You have a good point: Facebook has developed a really powerful and sticky social platform. While they explicitly want the primary experience on that platform to be around real world communications, there's no fundamental reason that the technology couldn't simultaneously be used for digital communities.

I feel strongly that the communities shouldn't overlap (I do not want to see pseudonyms in my comment threads), but the idea of a parallel Facebook where alternative profiles can interact with alternative profiles is interesting.