I would argue that my really close friends are people who I don't email much, but call instead. So your killer app would have to have access to my cell phone call history in addition to email history, and build the social network on a combination of calls and emails.
Right, I recognize that and agree that the top of the top is still phone calls, but as far as the web as a communication medium goes, e-mail is still the most used and most important.
Plus I couldn't immediately think of a non-intrusive way to collect call history from any phone so that idea was out ;-)
