Four hours after learning about Saturday's devastating earthquake in Nepal, I received a Facebook notification I had never seen before: Sonia, a journalist friend based in northern India, was "marked safe." An hour later, the same notification about a different friend popped up. Then another. Soon, several of my friends wrote that they, too, had learned via this strange new notification that their friends in Nepal were okay.

A few hours later, the mystery was solved. On Saturday afternoon, Facebook CEO Mark Zuckerberg announced on his timeline that the notifications came from Safety Check, a service the company launched last fall. "When disasters happen, people need to know their loved ones are safe," he wrote, "It's moments like this that being able to connect really matters."

A screenshot of Safety Check

When activated, Safety Check locates Facebook users near a disaster site through the city they list on their profile, or from where they last used the Internet. Users then receive a notification asking to confirm that they're safe or to say that they weren't in the affected area. Those who choose "safe"  generate a notification to their friends and followers, who can track how many of their friends were affected.

The idea for Safety Check emerged after a devastating tsunami struck Japan in 2011. "During that crisis we saw how people used technology and social media to stay connected with those they cared about," Facebook wrote when introducing the service.

Saturday's earthquake in Nepal, however, revealed some of the limits to Safety Check. Smartphone penetration in the country—one of Asia's poorest—is low, and six Nepalese out of seven are not registered on the social network. Electricity in the country is unreliable even during normal times, and there were reports of extensive power outages throughout Kathmandu in the hours after the quake.

But for those who can and do use Facebook, Safety Check's existence could offer an easy way for people to tell their friends and family that they're okay.

"At this time of desperation and disaster, just knowing your loved ones are safe is just like a beam of light in the dark," wrote Facebook user Dinesh Gurung in a comment posted beneath Zuckerberg's.