There's been a lot of talk about Facebook not being cool with kids anymore, and skewing towards an older demographic (anecdotally it's the older people I know who are the biggest users), but the privacy aspects seem to be having an impact too:

Beyond disruptive technology, Facebook’s creeping irrelevance among youth may have something to do with sheer perception. Francesca Johnson, a Grade 7 student at Branksome Hall—an independent school for girls in Toronto—isn’t currently allowed by her parents to sign up for Facebook, but she’s not entirely sure she ever will (she uses Instagram instead). Francesca says the site’s dubious privacy settings make her uncomfortable. She recalls one Facebook-heavy social media lecture she heard at school: “They told us a story about how one person said something bad about someone and he didn’t get accepted into college. I don’t want that to happen to me.” Francesca’s older sister, 15-year-old Anna (who prefers Instagram to Facebook, as well), thinks that some youth have moved away from the site, or stopped checking it as frequently, because parent-teacher scare tactics are more effective than we often think. After listening to the kind of social media lecture described by her sister above, Anna says she’s seen classmates “actually go on [Facebook] and change their privacy settings. One or two people even deactivated.”