Facebook chief executive Mark Zuckerberg on Friday promised to review the social network's policies that led to its decision to not moderate controversial messages posted by US President Donald Trump. The announcement, which came in the form of a letter to employees, appeared aimed at quelling anger inside the company that was so severe it prompted some to quit. The outrage was sparked when Zuckerberg said Facebook would not remove or flag Trump's recent posts that appeared to encourage violence against those protesting police racism. Zuckerberg's message Friday seemed to attempt to mollify that anger: "We're going to review our policies allowing discussion and threats of state use of force to see if there are any amendments we should adopt," Zuckerberg wrote. This, he said, includes "excessive use of police or state force. Given the sensitive history in the US, this deserves special consideration."