If you were an early adopter of FB it was exclusive to college students and had a much, much more "mature" audience to myspace. That factor alone attracted a huge crowd that myspace couldn't touch. In terms of UI and whatnot, myspace never really forced/required users to use their real name which I also feel gave FB a huge advantage going forth. The rest of the UI myspace could have copied though (but by that time it probably was too late).

Myspace was designed from the get go to be sold to a big company so it has little inherent value.

Rupert Murdoch bought Myspace for quite a sum as an attempt to get into the online business because all their attempts failed miserably for a lack of understanding and vision. The same drove myspace to its expected outcome.