Facebook is buying mobile messaging service WhatsApp for up to $19 billion in cash and stock in the firms biggest ever acquisition.
The world's biggest social networking company said 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.
'The acquisition supports Facebook and WhatsApp's shared mission to bring more connectivity and utility to the world by delivering core internet services efficiently and affordably,' the social network said.
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.
'The combination of WhatsApp and Facebook will allow us to connect many more people round the world,' Zuckerberg said.
'We want to develop more mobile experiences like Instagram and Messenger.
'WhatApp fits this vision perfectly, it has incredibly strong engagement and growth.'
'It's the only app we've ever seen that has grown more quickly than Facebook itself.'
'Their product roadmap is very exciting and won't change.
'Overall, I'm very excited about this deal.
'WhatsApp had every option in the world, so I'm thrilled they chose us.'
As part of the deal, WhatsApp co-founder and Chief Executive Jan Koum will join Facebook's board, and the social network will grant an additional $3 billion worth of restricted stock units to WhatsApp's founders, including Koum.
Experts say the deal could help Facebook attract teens.
Professor Will Stewart from the Institution of Engineering and Technology (IET) said: 'Obviously WhatsApp adds instant messaging to Facebook which must strengthen their position for the moment.
'Equally, all app types rise and fade, so established social media formats like Facebook will be overtaken by something new, and picking up candidates that might have ultimately replaced them may be a good survival strategy for a while.
'This does show the growing significance of mobile, though of course Facebook is on mobile anyway.
'But, Facebook has been around a while now so the real question is what comes next?'
