Many of the kids, too. In the "college bound" teen demographic, Facebook crushed MySpace. Since it was originally just for college students, when Facebook opened its doors to the pre-college crowd they began to swarm in.
This teen shift was just part of the growth of Facebook - they have continued to add members in all age brackets. Facebook's dominance grows as the "network effect" comes into play - if I'm a newbie to social media, I'm going to join the network where most of my friends are.