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.