I'd attribute it to two reasons, one because of Myspace and one because of Facebook.
First, Myspace allowed (and encouraged) aliases, whereas Facebook requires real names in profiles. It's tough to find your friends or your friends to find you when profiles are made up of users like "Sugar_Lady21" and "GymDude411."
Second, Facebook's rollout was gradual - it started college only (college by college), then companies, then high school, then anyone. It created envy among those who were not in the "cool group" of Facebook users, and the ones who were allowed viewed it as an exclusive club. When they were allowed to sign up, they were met with an already-large network of people to interact with. It was a brilliant growth strategy (even if unintended - it was originally meant to manage the growth and their available resources). People soon forgot they even had Myspace profiles.
Very good points. Makes sense.
Those are two very good reasons. I tend to think that MySpace also suffered because of its format. Having allowed users to heavily modify the look of their pages, a lot of pages simply ended up looking ugly. I seem to recall hearing someone once say that MySpace is to social networking what Geocities was to web design. By contrast, Facebook pages are simpler, and much easier on the eyes, and it simply looks like a more grown-up site.
Meanwhile, although you could heavily alter the graphical appearance of a MySpace page, there was only so much you could do to personalize the content. Photos are still hidden off the front page of a profile. So are blog posts. If you want noticeable updates and personal insight in your social networking, Facebook had MySpace beat by a mile.