A friend pointed me to this great post by Danah Boyd on Alternet discussing the implications of the race and class demographic divides between Facebook and Myspace: 

http://www.alternet.org/media/142356

The article starts off with observations about the manner in which the demographic differences between Facebook (affluent/white) and Myspace (less educated/minority), asks some challenging questions about the perceived "threat" of Myspace by schools and parents and then goes on to discuss the implications in terms of online campaigns, outreach and the perception of the online social sphere in light of the fact that, when it comes to social media 

the Internet mirrors and magnifies pre-existing dynamics. And it makes many different realities much more visible than ever before.

Some other choice quotes:

The data have consistently shown that MySpace is not a site of increased risk for youth and that risky behavior is more likely to occur in chatrooms than on MySpace. Yet, if you're a parent of a teen in this room, you're probably scared shitless of MySpace.

Why? What are you scared of? Are you scared of the site, or the possibility that your child might be exposed to values that are different than yours? Are you scared of the display of sexuality, or just the display of working-class sexuality? Needless to say, that's a topic for a whole different conversation.

and

...here's the main issue with social divisions. We can accept when people choose to connect to people who are like them and not friend different others. But can we accept when institutions and services only support a portion of the network? When politicians only address half of their constituency? When educators and policy makers engage with people only through the tools of the privileged?

When we start leveraging technology to meet specific goals, we may reinforce the divisions that we're trying to address.

If you want people to connect around politics and democracy, information and ideas, you need to understand the divisions that exist.

At the end of the article there are two great links, one to the dissertation from which it was taken and the other to a quantitative study on which much of the analysis was built. You can see a brief summary of the substantial demographic differences in Table 4 of that study.