Every site has a brand in the collective opinion of your teen's peers – ask and your kids can most likely tell you which social network is for professionals, older people, or younger teens. What has your kids safety to do with that? (They can probably also name the social networks where they are most likely to be approached by shady characters, scam artists, and pedophiles.) And when it comes to what's hot or what's popular, social networks are constantly evolving.
When social networking seriously appeared on the market in 2003, Friendster was the emergent king. It was one of the first of its kind to really take root, and became extremely popular. Everyone who was anyone was on Friendster.