lack of creativity in my humble opinion. Same deal with TV: Marketing guys have been sitting around scratching their heads as to why the 580thousandth time a fast car or kid on a skateboard being broadcast over frineds did'tn sell some candy or such. The usual BS is not super effective for younger people, and the marketing brass is hesitant while they figure out how to target kids. They are paying attention, they just arenet sure how to use the super valuable younger audience yet.
Google hit it off by using targeted searches, and associations with icons have always worked, both of which are possible for most social networks. I suspect this sector will pick up when an effective method of marketing emerges. Assuming its compatible with the existing social networks.