Facebook has bough the smartphone messaging service WhatsApp in a deal worth around $19 billion.

The social media giant will pay $4 billion in cash - the remaining 12 billion will be paid in Facebook stock.

Facebook is the world's biggest social network with more than 1.2 billion members.

WhatsApp with 45 million users will be operated independently with its own board.

It fits with Facebook founder Mark Zuckerberg's focus on being at the centre of  lifestyles in which billions of people around the world share whatever they wish over the Internet using smartphones or tablets.

It is Facebook's biggest acquisition and comes less than two years after the California-based Internet star raised $16 billion in the richest tech sector public stock offering.

Zuckerberg said that WhatsApp -- a cross-platform mobile app that allows users to exchange messages without having to pay telecom charges -- was worth the steep price because its blistering growth around the globe has it on a clear path to hit a billion users and beyond.

"Services with a billion people using them are all incredibly valuable," Zuckerberg said while discussing the purchase price during a conference call with analysts.

The deal came from a chat Zuckerberg had with WhatsApp co-founder Jan Koum, whom he described as a "valuable thought partner" and friend of many years.

Silicon Valley-based WhatsApp started the year with 50 employees, most of them engineers, and the startup said that all of its stakeholders have approved the take-over.

The purchase includes $12 billion in Facebook shares and $4 billion in cash. It calls for an additional $3 billion in restricted stock units to be granted to WhatsApp founders and employees that will vest over four years.

 Adrian Weckler is Technology Editor with the Independent group - he's been explaining the thinking behind the mega deal.