That's a narrow margin, but a significant one. MySpace is slightly older than Facebook and had an easier time opening up to the public. MySpace was also purchased by News Corp in 2005, giving them a huge backing, but despite that Facebook has still managed to grow significantly in the past few years. Most of that growth is coming from outside the U.S., which goes to show that Facebook's method of “translating” sites as opposed to MySpace's method of “localizing” sites is effective.
Facebook's growth is cited as being “steady”, not terribly fast, so it seems likely they will continue to grow in popularity with little difficulty.