This logic is fallacious.
> In just a week at UPenn, our startup Greekex has
> received 500 registrations via Facebook Connect
>(we do not offer an alternative login).
Any idea of how many didn't sign up after visiting, or backed out?
You have no alternative login so you can't compare it against people opting for those.
How about projected signups? Were your expectations met or exceeded?
All you're saying here is that you've had 500 signups through Facebook. No useful conclusion can be drawn from that.