But by 2019, Mr. Zuckerberg was champing at the bit to assert more control over his company’s apps, tying them together so they would share data and technology. That led to the departures of WhatsApp’s founders and other employees. Mr. Acton joined a rival company, Signal; Mr. Koum now focuses on philanthropy and buying high-end, air-cooled Porsches. Some former WhatsApp executives later accused Mr. Zuckerberg of breaking promises he had made concerning privacy when he bought the company.

Mr. Zuckerberg has since built WhatsApp into a more fully fleshed-out messaging service and business. WhatsApp has added more features, ranging from simple emoji reactions and message forwarding to disappearing messages and supporting the app across other devices, like Macs and Windows desktop computers.

For most of its life, WhatsApp had been more popular with users outside the United States. But with the new features, more Americans began trying the app. In the United States, it has grown fastest with young people in Miami, New York, Los Angeles and Seattle, according to the company’s studies. A Snapchat-like feature that allows users to post temporary text, photo and video updates, called Status, has become the world’s most used Stories product, Meta said.

WhatsApp also began offering paid tools and custom apps for businesses that wanted to use the platform to communicate with consumers. Chevrolet, Lenovo, Samsung and L’Oreal now use some of those tools, and WhatsApp has forged business and advertising partnerships in Latin America and India with companies such as Amazon and Uber.

In 2017, WhatsApp introduced “click-to-message” advertising, which is an ad format that businesses can buy to place inside a Facebook feed. When users click on the ad on Facebook, it links them to a brand’s WhatsApp account, where they can talk with customer service representatives or take an action like booking a flight or buying goods. The ads have become Meta’s fastest-growing ad format, the company said.