"This is one reason why there’s been some success in keeping extreme content like ISIS-related pro-terrorism posts and child pornography off the big platforms—the tech companies operate a shared database of flagged terrorist content, and can in many cases tag and remove hateful content before anyone sees it. (Though nonprofits like the Counter Extremism Project say it’s not as many cases as they’d like you to think.) But then there’s the mounting list of seemingly obvious failures: The terrorist videos Theresa May excoriated Facebook for in the aftermath of the 2017 London Bridge attack. The Christchurch shooting videos, which were allowed onto Facebook 20 percent of the time—making for hundreds of thousands of posts—when users shared them on the platform, according to the company."