Facebook is buying mobile messaging service WhatsApp for up to 16 billion dollars.

Social networking giant Facebook said that it will pay 12 billion dollars in Facebook stock and 4 billion dollars in cash for WhatsApp, a real-time mobile messaging service, by far the company's largest acquisition.

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

Facebook says it is keeping WhatsApp as a separate service and CEO Mark Zuckerberg says WhatsApp is on path to reach a billion users.