First, I don't think anyone should take anything on Facebook seriously. As my dad says, arguing with idiots only brings you down to their level and then they beat you with experience
To get all Freudian on you, I think the Id and Ego play a big part in it. People are inherently selfish or self-centered (interested in only what benefits themselves the Id), and Ego tries to make sense of something (in this case) they have no true understanding or knowledge of. Yet they want to be helpful. To get all Yogi Bear on you, everyone feels they're "smarter than the average bear" and must tell you how to fix it. They gave their answer at most several seconds of thought before giving it to you, so I'd give it the same amount of thought and no more ...
Annoying, yes but I just tend to laugh it off rather then get all bothered by it...