Teens are leaving Facebook because everyone and their grandmother is on it. They don't want everything they say and do to be seen by their family or the wrong people. I'm 33, so I can't say for sure, but that's what I'd think if I were 15.
I have friends, family, and colleagues on there. Because of that last part, by necessity, I can't leave Facebook. There are entire communities on Facebook that aren't going to just disappear.
It's like TrekBBS. Look at this place. It's been around since 1999.