A six-month research project has revealed a sharp division along class lines among the American teenagers flocking to the social network sites.
The research suggests those using Facebook come from wealthier homes and are more likely to attend college.
By contrast, MySpace users tend to get a job after finishing high school rather than continue their education.
The conclusions are based on interviews with many teenage users of the social networking sites by PhD student Danah Boyd from the School of Information Sciences at UC Berkeley.
In a preliminary draft of the research, Ms Boyd said defining "class" in the US was difficult because, unlike many other nations, it did not map directly to income.
Instead, she said, class in the US was more about social life and networks - how people define themselves and who they define themselves with.
"Social networks are strongly connected to geography, race, and religion; these are also huge factors in lifestyle divisions and thus 'class'," she wrote.
Broadly, Ms Boyd found Facebook users tend to be white and come from families who are keen for children to get the most out of school and go on to college.
By contrast, the average MySpace teenager tendeds to come from families where parents did not go to college, she said.
Ms Boyd also found far more teens from immigrant, Latino and Hispanic families on MySpace as well as many others who are not part of the "dominant high school popularity paradigm".
"MySpace has most of the kids who are socially ostracised at school because they are geeks, freaks, or queers," she said.
Teenage users of both sites have very strong opinions about the social network they do not use, she noted.
Ms Boyd was wary of drawing too many conclusions from her research and calling Myspace "bad" or Facebook "good" or condemning social networks out of hand.
She wrote: "This division is just another way in which technology is mirroring societal values."
In some ways, Ms Boyd wrote, social networking sites are helping teenagers cope with the stresses of 21st Century life.
"Teens are using social network sites to build community and connect with their peers," she said. "And through it, they are showcasing all of the good, bad, and ugly of today's teen life."