When Facebook bought WhatsApp for more than $19 billion in 2014, Jan Koum, a founder of the messaging company, arranged to sign a part of the deal outside the suburban social services center where he had once waited in line to collect food stamps.

Mr. Koum, like many in the tech industry, is an immigrant. He was a teenager when he and his mother moved to the San Francisco Bay Area in the early 1990s, in part to escape the anti-Semitic tide then sweeping his native Ukraine. As Mr. Koum later told Forbes, his mother worked as a babysitter and swept floors at a grocery store to survive in the new country; when she was found to have cancer, the family lived off her disability payments.

Tales of immigrant woe are not unusual in Silicon Valley. But Mr. Koum’s story carries greater resonance because his app has quietly become a mainstay of immigrant life. More than a billion people regularly use WhatsApp, which lets users send text messages and make phone calls free over the internet. The app is particularly popular in India, where it has more than 160 million users, as well as in Europe, South America and Africa.

Because it’s free, has a relatively good record on privacy and security, and is popular in so many parts of the world, WhatsApp has cultivated an unusual audience: It has become the lingua franca among people who, whether by choice or by force, have left their homes for the unknown.

This is happening as the world is increasingly at war over migration; 2016 was, among other things, a prolonged and pitched battle over the rights and privileges of migrant people, whether Syrians in Europe, Europeans in Britain’s fight over Brexit, or the issue of Mexican and Muslim immigration that dominated the American presidential race.

Beyond the headlines, what has often gone unnoticed in the politics of migration are the shifting dynamics of migrant life — particularly the surprising and subtle ways in which technology, especially smartphones and social networks, has altered the immigrant experience.

Immigrants use lots of different apps, of course, from Facebook to Skype to WeChat, which is popular in China. But for many, WhatsApp has been at the center of a newfound connectedness. Wherever there are people leaving their homes for uncharted shores, you are likely to find WhatsApp. For migrants, it has become the best way to stay connected along a route, or, once they have landed, to keep in touch with the people they left back home.

Syrian refugees flooding into Europe have used WhatsApp to pass along tips, warnings and pleas for help to others along the journey. WhatsApp has turned up along the border between the United States and Mexico, where Donald J. Trump would like to build his wall. In the last year, a tide of Venezuelans has landed in Miami. The first thing many of them reached for when they landed was WhatsApp.

Even for people who have left their home countries voluntarily to pursue jobs and wealth in a new place, WhatsApp has thoroughly altered the contours of immigrant life. People who have been in the United States for decades told me that WhatsApp has eased the feeling of isolation and longing that is inherent to being an immigrant.

“I do have much more of a sense of their daily lives,” said Anne Reef, 55, a former English professor, who moved from South Africa to the United States in 1988. She now lives in Memphis and has taught at, among other places, the University of Memphis.

Calling internationally was a costly affair during Professor Reef’s earliest days in America; she would rely on a once-a-week phone call for news from home. There were often letters, sometimes containing pictures of newborns and weddings. Faxing became a thing in the 1990s, and later she found email, Skype and Facebook. But it wasn’t until she started using WhatsApp about a year ago that Ms. Reef began to feel a qualitative change in her connection with her far-flung family.

A relative who lives in Australia — the son of Ms. Reef’s first cousin — recently had a baby. With WhatsApp, Ms. Reef said, she gets to see a stream of baby pictures. “I feel much more involved with the baby’s life — I feel like I know him, and that he’s become more than a third cousin to me,” she said.

 

This might sound pedestrian; after all, baby pictures on the internet aren’t revolutionary. But WhatsApp’s innovations tend to be subtle. One of the secrets to WhatsApp’s growth has been a focus on simplicity. The app is purposefully unflashy, and it does just a few things — texts, voice calls and video calls. As a result, it is supremely easy to use even for people who are neophytes to digital technology. This is one reason immigrants find it so powerful; it has given them access to a wider set of relatives who might have shunned the social networks that came before.

Adoption of WhatsApp often follows a curious pattern — older relatives often suggest it to younger ones, rather than the other way around.

“My aunt, who’s in her late 70s, was the one who really pushed me to get on it,” Ms. Reef said. Now, she said, she uses it nearly every day; lately she’s even gotten her children to use it.

WhatsApp’s ubiquity is also important. Because it has become essentially the primary mode of communication between people back in the motherland — whether your former home is Bangalore, India; São Paulo, Brazil; Johannesburg or Paris — for people who leave, it becomes something like a window into an old life.

“I have a group that has my mother’s side of the family, and then another group that is my husband’s side of the family, and all day long it’s just messages to each other,” said Mina Mehta, 65, a surgical technician in Chicago, who moved to the United States from India with her husband in 1975.

“It’s this constant feedback of news from people back home, and you get to hear about parts of their lives that they wouldn’t have mentioned in a once-a-week phone call,” Hemant, Ms. Mehta’s son, told me.

For migrants who leave their homes out of desperation, WhatsApp offers another advantage that many other networks lack: It’s secure. The app is encrypted, making it safe from government snoops. The company has also long been adamant about its opposition to advertising and some of the intrusions on privacy, though that stance has softened since the Facebook purchase. WhatsApp said in August that it would begin matching its users with those in Facebook’s database, a move that prompted an outcry from some privacy advocates (and that may result in a fine from European Commission).

Still, for Syrian refugees, WhatsApp is seen as the most secure communication tool that everyone uses, according to Majd Taby, a Syrian immigrant to the United States who spent a few weeks this year documenting the lives of refugees for a photo book he is making. Mr. Taby argued that without WhatsApp, the migrant flows out of Syria might have been much smaller.

“What WhatsApp did was demystify the journey,” he said. In the earliest days of the Syrian civil war, some of the first refugees leaving Syria faced a receptive welcome in European countries.

“People would have a group with their friends, and one of them would make it to the other side, and send them back a message telling them what the trip was like, and sharing photos,” Mr. Taby said. “That’s what caused a lot of people to decide to do it. They’d seen exactly what was going to happen on WhatsApp.”

WhatsApp itself does not keep tabs on how the app has been embraced by immigrants; a spokeswoman told me that because communications on the app are encrypted, the company has no way to know when people are communicating internationally.

But Mr. Koum, the company’s chief executive, said in an email that immigrant users are an important constituency.

“A lot of us at WhatsApp were born in other countries,” he said. “We recognize how important it is for people to connect with family thousands of miles away, because it’s something we think about a lot.” Every feature in the app, he added, “was designed in part by someone living the immigrant experience every day.”