I fully agree with you: You can't check with facebook whether a name is legitimate or not.
But, as I said in my post, you can get at a probability whether a name currently is in actual use anywhere, which in many circumstances is an interesting info in itself.
Anyway, this seems to be an interesting and almost philosophical question to me: If you have a name that was in use several centuries ago, and you have no idea whether it is still in use, is it legitimate then, and if yes, in which sense?
Pepin, for just one silly example, was a perfectly legimitate name during the times of the Merovingians, in the year 700 or so, in their kingdom that included large parts of what is Germany today. Does this make Pepin a legitimate German name until today?