MySpace failed due to many reasons (marketing, probably the primary reason), whereas, Facebook was a success due to (initial) exclusivity.
Also, MySpace was horrible in that they allowed (nearly) arbitrary HTML code for their design. These types of skins are non-invasive and don't change much of anything, as you point out.
Great additional points there, and I appreciate the vote @complexring - following.
In MySpace's defense, I did land a college aged girlfriend whom I dated for about 5 months before she had to move back to Serbia by being active on it, so it will always have a warm spot in my heart - thanks Tom!