I spent most of last weekend hanging around hundreds of teenage girls who, like me, were trailing around an up-and-coming teen heartthrob who got his big break on social media. I was there for a story, they for selfies, but while we sat cross-legged on the cement floor of a sports arena, they patiently humored my questions about which social networks they use. Twitter, Vine, Instagram, YouTube, the 13- to 16-year-olds answered. And you guys are all on Facebook, right? I asked my adopted clique, figuring it was such a given, theyd taken their membership there as assumed. No. Nope.