Facebook to buy messaging app WhatsApp for $19bn.

Its biggest acquisition was Instagram, which it bought for $1bn in 2012.Facebook has bought messaging app WhatsApp in a deal worth a total of $19bn (£11.4bn) in cash and shares.

It is the social networking giant's biggest acquisition to date.

WhatsApp has over 450 million monthly users and is popular with people looking to avoid text messaging charges.

In a statement announcing the deal, Facebook founder Mark Zuckerberg described WhatsApp's services as "incredibly valuable".

WhatsApp allows users to send messages over internet connections, avoiding text messaging fees. The company claims it is currently registering one million new users a day. It makes money by charging users a subscription fee of $1 per year, although it offers a free model as well.

The deal to buy it includes $4bn in cash and approximately $12 billion worth of Facebook shares, plus an additional $3 billion in stock to WhatsApp's founders and employees at a later date. Once the deal is finalized, Mr Koum and co-founder Brian Acton are set to become Silicon Valley's newest billionaires.