The "Paris Terror Attacks" safety check on Facebook lets people signal whether they were out of harm's way, then notifies all those they know at the leading social network.
"Quickly find and connect with friends in the area," a message at the Facebook Safety Check page read. "Mark them safe if you know they're OK."
The feature also allowed people to check which friends listed as being in Paris had not yet checked in as safe.
The death toll in the "unprecedented" series of attacks across Paris climbed through the night yesterday.