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.
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.
