I think teens would still be on Facebook if there were parts of it that adults couldn't access. Teens need autonomy from the grownups in the family. Facebook was never acceptable to the iGeneration (the teen children of older Millennials) because you couldn't hide from your parents and older relatives on there. Even if Facebook had made G+ style circles, what if you wanted to project an edgy image in your profile, and couldn't, because mum and dad could see it?