Pew Internet: Social networking sites and politics
Here are some interesting graphs about how liberals, moderates, conservatives deal with political differences they encounter on social networking sites (SNS). Seems liberals and conservatives or more likely than moderates to discover a friend has different political views than they expected. Liberals are a litlle more likely than moderates or conservatives to drop or block someone they disagree with. The more to the extremes you are the more likely you are to say that you agree with most posts by your friends, suggesting the presence of echo chambers.
I've never dropped a FB friend over political issues. I have dropped several twitter feeds from people (from both sides of the spectrum) who contiually tweet snarky political comments, and I've moved FB friends to categories in my feed where I see them less often. I enjoy discussing politics but echo chamber snarkiness is wearisome no matter which perspective it comes from.