Now that I think of it, at least four people I know have some sort of divorce or estranged-parent thing going on with their last names, and I never would have been aware of it if not for the mismatch between the name I met them under and the name on one of their online profiles.

You know the names they've given you.

You have no idea if those names are real enough for Facebook - plenty of people don't use the name on their birth certificate or marriage certificate or passport. (And this is legal and correct in many countries.)

Yes, like in California, where you're technically allowed to change your name through continued usage (though that doesn't fly for Social Security and thus neither for DMV or banks, which makes this method pretty useless)

http://www.courts.ca.gov/1054.htm

What I'm getting from this is that you don't know your "friends" particularly well. How do you know how well I know my friends?

I think you underestimate how common it is for people to use names other than on their birth certificates, passports or marriage certificates.

On the contrary, I am friends with such a person. We've talked about it.

Actually, I am friends with many such people, if you count Chinese people using English names. There is a mode where I don't know any part of a Chinese person's name, but there's no mode where I know their personal name but not their surname.