Among U.S. teenagers, Facebook (NASDAQ:FB) appears to be taking a back seat to more intimate social networks that have the ability to share with a smaller intended audience. Over the long term, this trend could prove problematic for a company that has just begun ramping up its business efforts. In this video, Motley Fool contributor Steve Heller explains why teenage trends matter and how they could ultimately threaten Facebook's business down the road. Steve also points outs why spending over $1 billion on Instagram last year was probably a good decision.