I got into a Facebook argument with someone I recently removed from my list, but every time I would counter with information about the subject we were arguing about (healthcare something or another), she would find a way to perform more elaborate mental gymnastics to invalidate or ignore the point I made.
Having a reasonable conversation means both sides need to be open to new information, and a lot of the time, that doesn't seem to be the case; especially considering how intolerant and radicalized some people's views are becoming. Regardless of what you tell them, they aren't going to listen, or will find a way to tell you that you're wrong.
This is why when I want to have a political conversation with a friend, I do it face to face over a beer. There's something about seeing your interlocutor in the eye when discussing matters that leads to more much open and honest discussion. The internet is the reverse - being behind a screen entrenches positions, closes minds and brokers dishonest conversation because no one wants to made a fool in public, especially with friends and onlookers watching on a public medium like Facebook. Besides NeoGAF, I almost never bother with any deep political conversation online. I just don't have the time.