I doubt anyone wants this. Facebook, Twitter, and Google are already brands. Nobody says, "I want to do social networking! I'm going to type social.com!" They just google Facebook and go there.
I agree with your guess. This kind of thing seems right up Microsoft's alley. They used to have an entire campaign using the phrase: "Welcome to the Social". If one of the big players goes in, I'm guessing Microsoft.
It seems like it might be perfect for LivingSocial. Still far from a household name with a somewhat awkward domain name (what does living social mean?), and lots of cash. I think they'd be happy to rebrand as social.com.