People seem to have an appetite for all kinds of heinous acts, and if you don't actively maintain a social standard that condemns and punishes such behavior, you're left with the law of the jungle.
Social networks already make it harder to maintain these standards because they dissociate their subjects from one another. It's easier for me to send you a death threat if, to me, you're just a bunch of words I disagree with under a profile picture.
They also facilitate the mobilization of like-minded people at an awesome scale, however widely unaccepted their behavior is. If you can get your sense of social belonging satisfied by 100000 other pedophiles from all over the world on a social network, why should you conform to any widely recognized social standard? Why should you work on your problems when thousands of rape apologists are patting your back and reinforcing your delusions?
Finally, social networks exploit these weaknesses, exploit our sense of pride and our ideas of what is right and wrong. More polarized and controversial information leads to more discussion. More discussion leads to more social data. More social data leads to better ads. There is an incentive for social networks to push objectionable and worthless information. The relationship is perhaps indirect in that it just happens to be the best way to turn a profit, but we shouldn't let our social standards budge for a system that is designed to maximize a profit margin.