So I said, narrow the focus. Your "use case" should be, there's a 22 year old college student living in the dorms. How will this software get him laid?
That got me a look like I had just sprouted a third head, but bear with me, because I think that it's not only crude but insightful. "How will this software get my users laid" should be on the minds of anyone writing social software (and these days, almost all software is social software).
"Social software" is about making it easy for people to do other things that make them happy: meeting, communicating, and hooking up.
Goddamn, jwz is in rare form in this entry.