Earlier quoted context omitted.
The WTC footage is documentation of a terrorist attack by innocent bystanders. The Christchurch Mosque shooting footage was created by the terrorist with the aim of becoming viral. It is a component of the terrorist attack itself.
The 9/11 attackers wanted attention, too, and they got it. Don't tell me that 9/11 wasn't carried out with the express intent to "go viral".
All the talk about social media is just a distraction for the real issues that politicians don't want to face: gun laws. We probably shouldn't be selling crazy people weapons of mass destruction. But sure, blame Facebook for letting people post videos...