How does Whatsapp make money?

Just why did Facebook buy social networking application Whatsapp for USD19B? With this amount of money you could buy several football clubs most of which are under USD2B. There must be a good reason why Facebook placed so much monetary value on this social network. The application costs £0.99 on iOS and other platforms and has no in-app purchases. There's no advertising on the platform which could have generated revenue. 

So how does Whatsapp work? It uses Internet data to send SMS messages across mobile phone networks across the world. All you need is a mobile phone number and a data subscription. At the time Facebook acquired Whatsapp it had more than 450 million customers and counting. Facebook basically got access to a big database of mobile phone numbers whose users have unique lives, needs, dreams and aspirations. They interact and make their preferences known. This is huge for big data analysts who can devise targeted advertising to the same customers on a different platform such as Facebook. Users of Facebook and Whatsapp most likely use the same mobile phone number to sign up for both Facebook and Whatsapp. 

Whatsapp has recently introduced a calling feature which enables cross network calling across the world.