Worrying about your parents being on the same social network as yourself ceases to be an issue when you become an adult yourself, and you no longer have to hide the fact that you were out partying when you were supposed to be studying at a friend's house. And as the GP notes, Facebook solves the problem that teens haven't yet had to confront - how do you catch up with your friends when you don't seem them several times a week?
In fact, the thing that has made Facebook such a phenomenon is not that teens found it cool, but rather that the much larger older population found it useful. I would be willing to bet that teens don't even make up 10% of Facebook at this point in time. When adults stop posting and stop receiving notifications on Facebook, then Facebook has something to worry about. Until then, my money would be on teens gravitating back to Facebook as they get older, and being cool becomes less important than knowing that all of your friends are contactable on Facebook.