According to a letter written by Senate Committee on Homeland Security and Governmental Affairs Chairman Ron Johnson, Orlando shooter Omar Mateen posted to Facebook during the massacre, and he also checked the social media site to see if the attack was trending. Johnson revealed this information in a letter to Mark Zuckerberg, which he also made public. The letter asks for Facebook's cooperation in investigating the attack.
Apparently, in addition to posting things such as "America and Russia stop bombing the Islamic state..I pledge my alliance to abu bakr al Baghadadi..may Allah accept me," Mateen also used Facebook to search for the terms "Pulse Orlando" and "Shooting" while he was still inside the club. As a reminder, there were approximately three hours between when the shooting began and when Mateen was finally killed by officers.