NEW YORK (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 and 70% of those people are active on a given day. 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.

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