Earlier quoted context omitted.
Perhaps because Facebook provides an easy and fairly reliable way to determine someone's sex. Many projects including dating sites have relied on it to do authentication for this reason.
I fail to see how it'd be good for this use - do they require a number of friends or something? I have 4 Facebook accounts for faking out exactly this kind of stuff, but none with any real information.
It's the most reliable "easy" way I know to determine this info. I remember an app called Lulu which did the same.