(AP) — Facebook is buying mobile messaging service WhatsApp for up to $19 billion in cash and stock, by far the company’s largest acquisition.

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

Facebook says it is keeping WhatsApp as a separate service, just as it did with Instagram, which it bought for about $715.3 million.

WhatsApp has more than 450 million monthly active users. In comparison, Twitter had 241 million users at the end of 2014.

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