The Pew Research Center's Internet & American Life Project has released The Demographics of Social Media Users—2012.
Here's an excerpt:
A late 2012 survey by the Pew Research Center's Internet & American Life Project shows that young adults are more likely than others to use major social media. At the same time, other groups are interested in different sites and services.
Internet users under 50 are particularly likely to use a social networking site of any kind, and those 18-29 are the most likely of any demographic cohort to do so (83%). Women are more likely than men to be on these sites. Those living in urban settings are also significantly more likely than rural internet users to use social networking.