Pew Internet & American Life Project. March 12, 2012.
It turns out that birds of a feather don’t always flock together on social networking sites when it comes to politics. As a rule, the most active and engaged political participants on SNS sit at opposite ends of the ideological spectrum, yet their experiences around political material on SNS are quite similar. Very liberal users and very conservative users are often the most likely to have acted for and against others on SNS. They are also more likely than others to have been surprised by their friends’ political views and to be in networks where they agree with what their friends post. Still, even with them, there is as much frequency of disagreement as there is of agreement. [Note: contains copyrighted material].
http://www.pewinternet.org/~/media//Files/Reports/2012/PIP_SNS_and_politics.pdf [PDF format, 16 pages].