I wouldn't have used the same language as the parent, but every time a developer creates a FB-only login, it promotes the idea that it is acceptable. It is not.
It really isn't acceptable to force someone else to sign up for one service to use a new service.
How many potential clients are they really excluding here? In other words how many people would use this service and not already have a facebook account?
I have one, but I categorically refuse to use Facebook to log in anywhere. In other words, if you look at how many have a Facebook account, you're looking at the wrong number.