For me, this article hits the nail on the head.

Facebook have purchased the future of mobile messaging by acquiring WhatsApp. People are calling each other less and less, so the primary communication function of a smartphone is becoming messaging, with many users upgrading to smartphones to bypass telecom companies subscription fees by using internet-based forms of communication.

Take China as an example, in 2008 , smartphones accounted for 9% of mobile phone sales, while in 2012 they made up 59% of the market. That takes yearly sales up to 307 million from under 200 million. I appreciate other factors are influencing this purchasing decision, but the externalities on the market resulting from this change in tech still hold.

Throw in the fact Mozilla are rumoured to be creating a $25 smartphone for emerging markets and it doesn't look like the demand for the service WhatsApp is providing is set to drop any time soon.

Another consideration is the decrease in the amount of time Facebook users are spending on the site, and the claims that Facebook is being perceived as 'uncool'. I can only speak for myself, but sharing my life on the site is something I rarely do. The only thing that keeps me coming back is ability to arrange events and message my friends, so Facebook have ticked one of those boxes with this purchase for me.