They already integrate with Facebook, just not prominently enough imho. And I don't know why Couchsurfing would cooperate since they're pretty much a competitor.
I guess I'm hoping for standards in this area to hurry up. Then you could do it between any major site and any other major site.
Kinda like linking accounts for easier sign-in (one site becomes the identity provider, the other becomes the relying party). But in this case, the purpose is to satisfy another end user of the validity of the claims, not the site a user is signing into.
