Most people use Facebook these days to keep track of their acquaintances from various stages in life: high school friends, college friends, former coworkers, people you meet at parties etc.

In that regard it’s not very useful to teenagers who are already in the same building as their entire network every day (their school). Plus teenagers don’t want to hang out where their parents do. When I was in school part of facebook’s appeal was the fact that parents couldn’t get on even if they wanted due to the .edu email requirement!