It is the social networking giant's biggest acquisition to date.
WhatsApp has more than 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.