And that's ok. But you'd like to test:

1) how many people would sign up without a facebook account? 2) how many of them could you "convert" and sign-in again with facebook? (saying something like "ok..now, how about you use your facebook account to [blabla] with your friends on facebook?

I'd say people are more willing to give it a try and see how it looks without passing facebook credentials first... That's just my hint..