yeah, it's cool right now. but, to get big they are going to have to let the 'unwashed masses' in. Expect more questions like, "why is paris hilton such a sloot?" and less about interesting topics with interesting answers.
Once that happens, and it will have to happen for them to have the number of users that twitter or facebook has it will descend into yahoo answer hell. I don't see how they avoid that and get mass user adoption.
That's very much a risk for Quora. But if they're still showing me my friends' activity that could ameliorate the problem. My friends are still going to be asking/answering questions that I am likely to be interested in.
Once that happens, and it will have to happen for them to have the number of users that twitter or facebook has it will descend into yahoo answer hell. I don't see how they avoid that and get mass user adoption.