While debunking blatantly obvious false claims on Facebook, I am bewildered by people who are so totally unwilling to accept any conflicting ideas no matter how much logical evidence is provided. Of course it often comes down to who's truth they are willing to accept disregarding any evidence that is contrary to their beliefs - because any opposing information must be fabrications. The problem is only compounded when people's false beliefs are a result of hearing the same thing so many times over the years that they become embedded and accepted as fact. One of my favorite new claims is that the urban legend debunking website Snopes has an obvious bias and will outright lie in some of their stories. If you mindfully examine these claims you will always find that Snopes provides more than adequate evidence and references to support "their side of the story" having to do with the exact information provided on any given topic - far more in fact than the false stories claim themselves and at the very least, plausible deniability. Most often they are ridiculed for bias not because their facts are wrong specific to any given story, but because they infringe on an illusion.
Cognitive dissonance theory is founded on the assumption that individuals seek consistency between their expectations and their reality. Because of this, people engage in a process called dissonance reduction to bring their cognitions in line with their level of comfort - reducing discomfort and restoring balance caused by alterations in ones attitudes, beliefs, or behaviors. This creation of uniformity allows for a lessening of psychological tension and distress. The most powerful form of dissonance reduction is achieved by simply ignoring or denying any information that conflicts with existing beliefs.