NEW YORK (AP) — Facebook is buying mobile messaging service WhatsApp for $19 billion in cash and stock, by far the company’s largest acquisition and bigger than any that Google, Microsoft or Apple have ever done.

The world’s biggest social networking company said Wednesday that it is paying $12 billion in Facebook stock and $4 billion in cash for WhatsApp. In addition, the app’s founders and employees — 55 in all — will be granted restricted stock worth $3 billion that will vest over four years after the deal closes.

The deal translates to roughly 9 percent of Facebook’s market value. In comparison, Google’s biggest deal, Motorola Mobility, stood at $12.5 billion, while Microsoft’s largest was Skype at $8.5 billion. Apple, meanwhile, has never done a deal above $1 billion.

The price stunned Gartner analyst Brian Blau. “I am not surprised they went after WhatsApp, but the amount is staggering,” he said.

Facebook likely prizes WhatsApp for its audience of teenagers and young adults who are increasingly using the service to engage in online conversations outside of Facebook, which has evolved into a more mainstream hangout inhabited by their parents, grandparents and even their bosses at work.

“This is a bet on the future for Facebook,” Blau said. “They know they have to expand their business lines. WhatsApp is in the business of collecting people’s conversations, so Facebook is going to get some great data.”

In that sense, the acquisition makes sense for 10-year-old Facebook as it looks to attract its next billion users while keeping its existing 1.23 billion members, including teenagers, interested. The company has said it will develop a “multi-app” strategy, creating its own applications that exist outside of Facebook and acquiring others.

“Facebook seems to be in acknowledgement that people are using a lot of different apps to communicate,” said eMarketer analyst Debra Aho Williamson. “In order to continue to reach audiences, younger in particular, it needs to have a broader strategy…not put all its eggs in one basket.”

Facebook said it is keeping WhatsApp as a separate service, just as it did with Instagram, which it bought for about $715.3 million in two years ago.

WhatsApp has more than 450 million monthly active users. In comparison, Twitter had 241 million users at the end of 2013.

Facebook CEO Mark Zuckerberg says WhatsApp is on path to reach a billion users.

“The services that reach that milestone are all incredibly valuable,” Zuckerberg said.

WhatsApp, a messaging service for smartphones, lets users chat with their phone contacts, both one-on-one and in groups. The service allows people to send texts, photos, videos and voice recordings over the Internet. It also lets users communicate with people overseas without incurring charges for pricey international texts and phone calls. It costs $1 per year and has no ads.

The deal is expected to close later this year.

Shares of Menlo Park, Calif.-based Facebook slid $1.12 to $66.94 in extended trading after the deal was announced.

__

AP Technology Writer Michael Liedtke in San Francisco and AP Business Writer Tali Arbel in New York contributed to this story.