Total speculation driven by personal anecdotal evidence:

I think a lot of teens don't have a use for Facebook yet. Facebook is a pretty good tool for staying connected with people from high school and college and past jobs that you wouldn't see otherwise. Most teens are in high school with the same people from their jr high/gradeschool. They have smaller social networks, are focused on small groups of immediate friends, and have watched the kids a year or two older than them get in serious trouble for posting the stupid stuff teens do on the permanent social networks (Twitter, Facebook), and would rather use ephemerality (Snapchat) to avoid accountability.