https://phys.org/news/2019-03-ways-social-media-platforms-content.html

But we should avoid scapegoating the big platforms. All of them (Twitter, Facebook, YouTube, Google, Snapchat) are signed up to the European Commission's #NoPlace4Hate programme. They are committed to removing illegal hateful content within 24 hours, a time period which is likely to come down to just one hour.

Aside from anything else, they are aware of the reputational risks of being associated with terrorism and other harmful content (such as pornography, suicide, paedophilia) and are increasingly devoting considerable resources to removing it. Within 24 hours of the Christchurch attack, Facebook had banned 1.5m versions of the attack video – of which 1.2m it stopped from being uploaded at all.