Americans who use social networks are more affluent than the national average

"If you’re in the U.S. and are using a social network like Facebook, MySpace or LinkedIn, chances are you’re more affluent and more urban than the average American according to Nielsen Claritas, which provides in-depth segmentation analysis of consumer behavior.
“Nielsen’s online data shows that about half of the U.S. population visited a social networking website in the last year and that number grows every quarter,” said Wils Corrigan, AVP, Research & Development, Nielsen Claritas. “The rising popularity of these sites and the deep engagement consumers have with them has advertisers and marketers asking for more and more detail as to which lifestyles should be targeted for their online advertising and promotions.”
Through Claritas, Nielsen defines U.S. households in terms of 66 demographically and behaviorally distinct segments like “Young Digerati” or “Beltway Boomers.” When those segments are overlaid with the activity of Nielsen’s online panel of more than 200K, we see a marked difference in the demographic makeup of the two largest social networks, Facebook and MySpace.
Facebook users have a largely upscale profile. The top third of lifestyle segments relative to affluence were 25% more likely to use Facebook than those in the those in the lower third.
The bottom third segments related to affluence are 37% more likely to use MySpace than those in the top third.
Users of Facebook were also much more likely to use LinkedIn, a network geared towards business and professional networking, than those who use MySpace."
Source: NielsenWire blog, 25th September 2009