You should re-consider making Facebook your only option for logging in and signing up.
http://bijansabet.com/post/16980728547/why-facebook-connect-...
And the ensuing discussion, here: http://news.ycombinator.com/item?id=3548081
http://bijansabet.com/post/16980728547/why-facebook-connect-...
And the ensuing discussion, here: http://news.ycombinator.com/item?id=3548081
But requiring Facebook makes certain viral features (like sharing with your friends, posting to your wall etc) much more frictionless.
I think there are 3 ways a site can become popular. 1) Through viral channels. 2) Through word-of-mouth. 3) Through marketing/pr.
I'm going for all three. #1 is a lot easier if you require FB Connect.