In practice, it's probably even pretty close to 100%, given the login requirement. :)

In answer to my own unasked question, yes, there may be logins available on bugmenot (http://www.bugmenot.com/view/facebook.bug) although when you search for 'facebook' it says it says that it has been 'barred'.

It is your market. I am a teenager (guessing that is your market based on stats). I have a facebook account (don't use it much), but will absolutely never sign in to another site using Facebook. The security issues worry me, as does the access you have as a service to my facebook profile.

The fact that they have a facebook account does not mean that they want to use it to login. There is some loss from day one given your strategy.