Facebook activated its "Safety Check" function for the first time in the United States Sunday after the nightclub massacre in Orlando, Florida.
The Safety Check, first introduced in October 2014, allows Facebook users to notify friends that they are safe in the wake of a natural disaster or a crisis.
"Waking up this morning, I was deeply upset to hear about the shooting in Orlando. My thoughts and prayers are with the victims, their families and the LGBT community," said Facebook Chief Executive Mark Zuckerberg on his official account.
The shooting quickly emerged as a top trending topic on Facebook and Twitter, with hashtag #PrayforOrlando being one of the top posts throughout the day.
Last year, Facebook vowed to turn on the Safety Check more often during disasters in response to criticism that it enabled the function after Islamic State militant attacks on Paris and not the day before, when a bomb killed at least 43 people in Beirut.
Facebook has previously employed its Safety Check feature after emergencies in France, Belgium, Pakistan, Nigeria, India and other countries.
Facebook and Twitter were used by people at the scene of the attack at Pulse, the popular gay nightclub in Orlando. Club goers traded messages as they searched for loved ones in the chaotic aftermath of the massacre.