That seems like a pretty solid filter for me.

> But meeting someone doesn't necessarily mean you know their full name

Which is why Facebook's policies and technology is so great: I can easily find and add a new friend who I just met even if I don't have their last name yet.

> neither does it mean that they want you to know it

I can't plausibly imagine any real world scenario where I would willingly share my first name but not my last, but maybe that's just because my name is pretty identifiable.

> Also, how do you prove to Facebook that this is "the name you go by" if they challenge it?

That's the difficult part, and what Facebook is trying to find a good answer to.