We have some MySpace accounts for girls and they have had limited success, but I would say this is due to neglect.
Myspace pages do well with activity so if the page is a conduit for personality, then it can work.
Facebook has done better for us because the girls get more involved.
YouTube is great for exposure and some traffic, but be very careful with your submissions, it's a mainstream site so even the tiniest bit of nudity can get your video deleted.
Also, check out Rude.com (
http://www.rude.com/FreakySteve - still in progress) and xtube.com. They are more like YouTube/MySpace hybrids for porn. I like xtube. I'm still seeing how rude is going to do.