Sadly, Facebook isn't dull and sterile anymore. I can't spend a minute there without someone asking me to join their mafia or pushing some dumb "which care bear are you" quiz on me when I just want to chill out and exchange messages with my friends and comment on my fruit smoothie this morning.
So you are saying that black MySpace users tend to have a bad taste in colors and music, and therefore the white users leave? Whereas surely white users only create tasteful profiles?
Or maybe MySpace in general sucks, and therefore people are leaving irrespective of race.
One thing though: MySpace seems to be the standard site for music and bands, so perhaps "races" who are more preoccupied with music would tend to hang out there more.
Facebook maintains high quality and a coherent feel by realizing that most people couldn't design their way out of a paper bag, and only giving users very basic, high-level control over their page.
IMO, everything flows from that. MySpace feels like Geocities: total amateur hour. Even if you had the exact same group of people (and I do agree there are demographic differences between MySpace and FB, I just don't think they're that relevant), Facebook would still be easier and more pleasant to use.
To be fair, that's mostly dependent on the sort of company you keep, and you can turn off the notifications for all that sort of stuff anyway - it makes FB actually usable :)
Thing is, it's still true. Facebook is dull and sterile, MySpace is still fertile and full of musicians.. and yet now I'm on Facebook and never use MySpace. Go figure!