Interesting report from the Pew Internet & American Life Project, "Social media is slowly changing the demographics of political engagement," by Frederic Lardinois.
Here is an excerpt:
A far more interesting statistic is that 15% of all Internet users have left comments on websites about political or social issues, or posted images or written blog posts related to politics or social issues. What is even more interesting, though not surprising, is that young adults between 18 and 29 are far more likely to use social networks as a venue for political and civic engagement than older users. These younger users who engage in political activity online are also far more likely to participate in politics offline.