Most Facebook controversial posts aren't invitations for discussions, but biased rants.

Discussions almost never really take place. Most people don't know how to be objective.

You can make all of the arguments you want with every piece of evidence supporting it, yet most people are so entrenched in their way of thinking that they still refuse to really look at issues objectively. Even when slammed in the face with the truth, they'll generally find some weak straw man argument to still retreat to in order to maintain their biased point of view.