"Waking up this morning, I was horrified to hear about the shooting in Orlando. My thoughts and prayers are with the victims, their families and the LGBT community," Facebook CEO Mark Zuckerberg said in a status update.
"We’ve activated Safety Check, but the biggest need over the next few days will be for people to continue to donate blood," he added, with a link to more information.
The Facebook Safety Check was first used in 2014, so that people exposed to natural disasters could show their Facebook friends that they were safe. It was recently used following the earthquake in Myanmar on 13 April 2016.
However, following the terrorist attack in Paris in November 2015, when Daesh militants attacked a nightclub and cafes in Paris, killing 129 people, the Safety Check was activated so survivors could tell their friends and families if they were safe.
The Safety Check was also used following another terrorist attack in Brussels in March 2016, when 32 people were killed by suicide bombers.
Then, in the early hours of 12 June 2016, Omar Mateen, a 29-year-old US citizen murdered 49 people and injured 53 others with an automatic assault rifle and handgun inside Pulse nightclub in Orlando, Florida, in what has become the worst mass shooting in US history.
US President Barack Obama has declared the attack "an act of terror and an act of hate."
I have nothing 2 say that hasn't already been said on this the 163rd day of a yr in which there've already been 173 mass shootings in the US— Michael Moore (@MMFlint) June 13, 2016
#PrayforOrlando trended on Twitter and Facebook throughout the day, following the shooting, uniting the world in their sadness and shock at the mass murder by a man who had legally bought an assault rifle and handgun and who targeted an LGBT club in Florida.