Overtaking Myspace may take some time. But what's important for me, personally, is that Facebook proved it can be done. It proved that the fact that someone else is a market leader, and boasts amazing user numbers, doesn't mean you have to stay out his turf.
Facebook is a great example that no one is safe (not even Google) and that there is no market that isn't worth fighting for.
One more point. Facebbok is what I like to think of as web 2.0 with a web 1.0 User interface. It's so simple an intuitive, it isn't more difficult to use than the good old Hotmail. This, compared with the messiness of Myspace makes it more appealing to "older guys" (30-40) .
Being appealing to this age segment may not shift the overall balance in user numbers but it may shift the "demographic" balance attracting to Facenbook more users that are decision makers and have much more money to spend than the average 16 years old.