You might make the same argument for only supporting IE as the only browser you app works with. I suspect it's actually an issue from day one as there are folks who would sign up but don't because you require Facebook.
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'.