I have been using MySpace since it's peak and have begun as of late to use Facebook as well. I am hearing of loads of teenagers and young adults slowing down their MySpace usage and going over to Facebook. I can see many reason's why mainly being that from a usability standpoint Facebook is 10 times better then MySpace. If you simply want to interact with friends and more importantly keep sub-groups and networks of people seperate Facebook is a fantastic application.
So the question is can Facebook become the new MySpace. I would argue yes but a few things might stand in the way. Where Facebook accells in usability and organization it does not allow you to completely personallize your page to reflect your personality graphically or musically. MySpace also accels in the music aspect of it's offering which was why it was created in the first place. I can see why Facebook is used more by the young adult crowd who as they get older care more about friend association then personaliation to express themselves.
Since Facebook has now opened its services up to everyone, before you had to be part of a college or corporation, I believe we will see a lot of people begin to start using facebook more. The chances that a former MySpacer will start to spend less time on MySpace and more on Facebook really depends if the MySpacer is tired of spam bulletins, useless friend requests, bad and forced marketing messages and more time filtering through crap then interacting with friends. I believe the now Corporate influence of MySpace will spell its demise to users who will find a fresh and very efficient Facebook the new MySpace.