Facebook has agreed to buy hugely popular messaging app WhatsApp for $19 billion (£11.4bn) in stock, cash and RSUs.

The move "accelerates Facebook's ability to bring connectivity and utility to the world" according to an SEC filing in the US which details the acquisition.

Facebook says the service will continue to operate independently of Facebook, and the social network's Messenger app in particular.

In a press release announcing the huge deal, Facebook CEO Mark Zuckerberg said: "WhatsApp is on a path to connect 1 billion people. The services that reach that milestone are all incredibly valuable."

WhatsApp has 450 monthly active users with 70% of those using the service every single day. It means that Facebook is paying approximately $40 for each of WhatsApp's users. WhatsApp currently has 32 employees who will continue to work at the company's Mountain View headquarters.

The purchase of WhatsApp for $19bn dwarves Facebook's other social messaging service pruchase - Instagram - which cost the social network less than a $1 billion.

WhatsApp co-founder and CEO Jan Koum will join Facebook's board of directors as part of the deal. "Doing this will give WhatsApp the flexibility to grow and expand, while giving me, Brian, and the rest of our team more time to focus on building a communications service that's as fast, affordable and personal as possible," Koum said in a blog post.

The CEO also addressed WhatsApp users' worries about any possible change to the service, saying they shouldn't worry about ads "interrupting your communication.

"There would have been no partnership between our two companies if we had to compromise on the core principles that will always define our company, our vision and our product," he said.

The deal is subject to regulatory approval in the US and should anything happen to scupper the deal, Facebook has agreed to pay WhatsApp $1bn in cash and $1bn in stock.