We know that people tend not to lie about their personalities on Facebook*. It seems they tell the truth about sex, too, or at least their intention to engage in it. "(D)isplay of sexual references on teens' Facebook profiles is associated with their intention to initiate intercourse."
Okay mom, now you know. If you monitor your kid's Facebook pages (I don't), when she** starts talking about sex, that's when you have the serious talk about sex.
*You know that I'm talking about normals here, right? There will be a certain number of crazies who lie because they're nuts, and psychopaths who lie because they're evil, and people using it for business or politics will say whatever it is useful for them to say. But most people using Facebook for personal reasons seem to use it candidly. This is what causes the problem for young people whose prospective employers find the drunken photos from spring break.
**I said "she," because I assume every teenage boy always has an intention to engage in sex, just no opportunity. Evolutionary theory says males have a more-or-less continual serial offer of sex with anyone, and females pick the one they want to mate with. This has not been my experience.