Myspace dominates US social networking sites. Indeed it dominates all US web accesses. As I noted in June, Facebook comes in 3rd among US users of social networking sites, far behind MySpace. But Facebook has nearly complete control of the college social networking market. And they appear to be leveraging that elite position very effectively.
Facebook started with incremental rollouts at US and then UK colleges. In September 2005 they added a separate site for high school students. In February 2006 they allowed college networks and high school networks to interconnect. Finally, a few months ago they started adding corporations. At every stage they have preserved a sense of exclusivity. For example, you must have an appropriate email address (initially a ".edu" address at a member school) in order to register. Of course once you are registered, you can substitute any other email address and, while you start as a member of the network at your college, you can search across all networks for people and invite them to be friends. So there's a strong sense of exclusivity or elitism, but not as much exclusivity as you think.
Fred Stutzman, a PhD student at the University of North Carolina, posts a very interesting analysis of Facebook adoption among incoming freshman at UNC. The first thing to note is that more than 90% of incoming freshman have signed up on Facebook by the beginning of the summer before they arrive on campus. This was true in 2005 and it is true now. What's different this year is that most incoming UNC freshman already have a large set of friends on Facebook that are not part of the UNC network. Here's the image from Stutzman's essay.
Obviously the high school program has worked wonders. Evidently anyone planning on going to college (at least to UNC), gets onto Facebook while they are still in high school.
Today Myspace gets five times the page views of Facebook, but of all the English language social networking sites (i.e. not counting sites like Cyworld), Facebook has the market position as the exclusive, I'd say elitist, service and they appear to be leveraging that position effectively.
Look for Facebook to move from #3 to #2 in short order.
Perhaps more important, when it comes to monetizing social networking services, Facebook's user demographics give them a strong position. (Some indicators here).