According to the company, some 4.1 million users checked in using the tool in the first 24 hours after the attack. That's roughly one-third of the 12.1 million people who live in the Paris metropolitan area. Those posts reached 360 million users worldwide.
That more than doubles the reach of posts made after an earthquake devastated Nepal in April; more than 8 million users checked in to Safety Check after that tragedy.
The Paris attacks were the sixth time that Facebook has used Safety Check. It had previously been deployed for the Nepal earthquake, as well as quakes in Afghanistan and Chile. The social network also deployed the feature after Tropical Cyclone Pam in the South Pacific and Typhoon Ruby in the Philippines, Alex Schultz, Facebook's vice president of growth, wrote in a company post.
Facebook was inspired to create the tool in the aftermath of the 2011 tsunami in Japan, and the company first launched the feature in the fall of 2014. At the time, Facebook said it would take factors such as the scope, impact and duration of disasters to determine when to activate the feature in the future.
During an ongoing crisis, like war or epidemic, Safety Check in its current form is not that useful for people: because there isn't a clear start or end point and, unfortunately, it's impossible to know when someone is truly “safe.”
That, along with the sheer reach of its European users, may explain why Facebook hasn't used the tool for other attacks, such as Thursday's bombing in Beirut -- a decision for which it faced a great amount of scrutiny. But the company said it's evolving its criteria for the tool and will use Safety Check more broadly for "other serious and tragic incidents in the future."
Facebook chief executive Mark Zuckerberg commented on his own page about the decision to deploy the tool for the Paris attacks and thanked users for their feedback.
"Thank you to everyone who has reached out with questions and concerns about this. You are right that there are many other important conflicts in the world," he said. "We care about all people equally, and we will work hard to help people suffering in as many of these situations as we can."