Teenagers wants fun. They want storytelling. It´s not happening on Facebook. And they are in an age where they absolutely are not craving that the whole wolrd knows what they do, who they are, and with whom they are playing with.
They are moving to messaging services, which are becoming social networks on their own.
Read more here:
http://aresonance.posthaven.com/facebook-vs-advertising
http://www.theguardian.com/technology/2013/nov/10/teenagers-messenger-apps-facebook-exodus