Yeah, Facebook started out with upper crust college kids. The kind of people that have money, and that marketers want. MySpace was a bunch of broke people. Facebook was way more legitimate as a business tool because of its clientele of college kids and better even, recent college grade.
If it was the fact of being a business tool that made fb overtake Myspace, then Linkedin should be kicking fb's ass. Facebook was designed for college kids while Linkedin was designed for business professionals. My company made all of us join Linkedin about a year ago. I've yet to find it useful for anything other than adding about a dozen or so useless emails to my inbox on a daily basis.