My politics, such as they are, are slightly to the other side of the line than most of the people in my Facebook contacts. A good number of those contacts are prone to posting what seem to me to be quite biased, divisive articles essentially preaching the moral superiority of the choir to the choir. My preferred style of engagement is to ask questions rather than assert "truths" and I try to steer clear of speculation on motive, appeals to authority, and all those rhetorical cop-outs. When I try to engage people on this stuff, the result is often quite hostile and sometimes personal. This makes me suspect that many people posting these things aren't really looking for debate, they're just looking for approval from their group. It would save me a lot of grief if Facebook provided a flag so people could indicate what kind of responses they're looking for when they post these things.
Having said all that, I find pretty much the same thing here on Slashdot and on most on-line fora. I just don't get the impression that many people see debate as a constructive way of testing one's beliefs and ideas.