As Michael Arrington recently posted, Ning's ousting of adult communities from their platform protects Ning's Google Ad Words business, but leaves these alt. networks out in the cold.

Interesting question: So, where will they go?

I'm tempted to say Tribe.net, which seems to be reinvigorating itself somewhat--but with a greater interest in monetization. Tribe has always been Burner heaven, so welcoming in some edgier adult groups might not be that great a stretch.  However, Tribe's new-found emphasis on monetization is all tied to google adwords (see the results of this front page query for an example of that), so there's a pretty short plank there.