Today, March 15, marks the fifth anniversary when an Australian gunman opened fire on worshippers at two mosques in Christchurch, New Zealand. The shooter livestreamed the killings on Facebook for 17 minutes before the video was taken down by the platform. Fifty-one Muslims died from the shootings. In livestreaming the murders, the gunman, then 28 years old, demonstrated a disturbing understanding of how such content is engineered to go viral. And go viral it did. The following day, Facebook issued a public statement: "In the first 24 hours we removed 1.5 million videos of the attack globally, of which over...