well not necessarily die by 2018, just the fact that social media is mostly moving fads. everyone wants to be the social network that high school and college girls post their vacation photos to, if you are that then you will likely be somewhat relevant for the lifetime of these girls.

Or maybe it's the opposite: if a teenager has an account with lots of pictures, he/she will probably want to kill that account as soon as they get to adult life to avoid awkward moments. I think this explains why many young kids are leaving FB.

The bigger issue is kids don't want to use the uncool social networks their parents use. Secondarily, that might come into play later as an afterthought.