Alternatively, rather than imprinting into a particular social network (Facebook bad, Twitter obscure, Instagram relevant) it could be that these teens will simply migrate to a different toolset when their needs change rather than expecting a tool to be used to photoblog their party lifestyle of college years to also work as a tool to keep up with post-college life. Facebook changed over time to basically follow a particular demographic (those whose age mirrored the founder) but most of the others are built for specific purposes and have a lot more inertia behind them. Maybe being first is what let Facebook change and re-define itself, but so far none of the others seem to have changed much from the day they were launched in terms of how people actually use them (and FB itself has had numerous failures in attempts to spin off mini-pivots as individual apps.)