The attacks on Paris that killed at least 129 and injured many more sent droves of people online to search for lost loved ones or for word of the safety of friends and family.

One social media post cited by the London Observer read: “Waleed is missing. We last contacted him at the match. Please share & contact me if u have any info. #rechercheParis.” Another said “I’ve been looking for my cousin since last night….He’s 25 and I’m 75. He’s called Younes. #rechercheParis.”

The hashtag #rechercheParis showed social media’s potential for connecting people after a tragedy, as did #porteouverte (“open door”), which Parisians used to offer safe houses to people fleeing the massacres.

Facebook turned on its Safety Check feature to help people in the French capitol check on loved ones, and millions of people used it to let friends and family know they were safe, according to an article by Reuters news service. The feature was launched in October 2014. This is how it works: After a disaster such as an earthquake, Facebook sends out a notification asking if users there are safe. If a user marks “I’m Safe,” a notification will be sent out to the user’s feed.

facebook paris security check(Photo: SafeBee)

Related: 3 Emergency Apps Everyone Should Have on Their Cell Phones

Critics said Facebook should have made Safety Check available during other attacks around the world, such as the suicide bombings in Beirut that killed 43 people the day before the Paris attacks. Facebook Chief Executive Officer Mark Zuckerberg said those questions were justified.

"Many people have rightfully asked why we turned on Safety Check for Paris but not for bombings in Beirut and other places," he wrote on his official Facebook account. "Until yesterday, our policy was only to activate Safety Check for natural disasters. We just changed this and now plan to activate Safety Check for more human disasters going forward as well."

Other tools for finding loved ones include Google Person Finder, an open source web application that serves as a registry and message board for survivors and loved ones, and the Red Cross’s Safe and Well website for locating people after a disaster. You don’t even need Internet access to use it. Text the Red Cross or call 1-800-RED CROSS and select the prompt for "Disaster" to register yourself and your family.

Related: Mass Evacuations: How to Flee a Disaster Area

Such tools have made huge advances over the last 15 years, says Russ Paulsen, the executive director of community preparedness for the American Red Cross. “As I talked to people after Hurricane Katrina, one of the worst things I heard was families who were separated for weeks and had no idea if their family members were alive,” said Paulsen. "It was heartbreaking."

Think now about how you would get in touch with every member of your family after an emergency, he advises.

Related: Preparing for a Disaster: Tips from the American Red Cross

Diana is an award-winning writer and editor with more than 20 years' experience in magazine, video, book and digital journalism, with a specialty in health coverage. She was a longtime writer and news editor at the Center for Investigative Reporting; has written for publications from the Washington Post to the Times of London syndicate; and has served as a senior and/or consulting editor at Time Inc. Health, Hippocrates, HealthDay News Service and Reporting on Health. She was also editor in chief of Consumer Health Interactive, a national health and medical web site, and has reported on finance for Blueshift Research and PBS Frontline. Before joining SafeBee, she was editor of Bioenergy Connection, a national magazine about bioenergy at UC Berkeley. Her favorite safety tip: Wear a bike helmet.