You cant buy love and the is worth 19 billion

This best explains the heavy price tag on WhatsApp or any other blockbuster acquisitions (e.g. Instagram):

As a programmer if you ask me if WhatsApp is worth that much I’ll answer expletive no! It’s just sending stuff over a network. Get real. But I’m also the guy that thought we don’t need blogging platforms because how hard is it to remote login to your own server, edit the index.html file with vi, then write your post in HTML? It has taken quite a while for me to realize it’s not the code stupid, it’s getting all those users to love and use your product that is the hard part. You can’t buy love.

The rest of the at highscalability goes into more interesting details, so if you get a chance read it. But now you know why Viber was acquired for "just" 900 Million.