Here is a couple of theories I have had about it:
Firstly, Facebook doesn't need to monetize it. WhatsApp seems to be generating enough cash to run on its own. Instead, WhatsApp is a data-cow (I, phantom_oracle, officially coin the term data-cow on 21/2/2014 :P, which is the 21st century equivalent of a cash-cow).
WhatsApps value proposition lies in the data it has access to. I've already read on pandodaily/wired/one of those tech blogs that explains that whatsapp can access your private data, as it is in their terms.
So now think about it.
WhatsApp is always on/connected, so user tracking and trend-creation becomes easier.
People discuss a lot of things on it too, thus a lot of personally identifiable info exists (think, planning meals with your loved one, chatting to your kids, etc.).
By combining these 2, FB can improve its targeted ads on the FB platform and Instagram. Link all three and they basically have the equivalent of a user-database that is the envy of the NSA (pictures, messaging, location, etc, etc.).
If FB can improve its ad-revenue by even US 1 billion each year through this purchase, then the real ROI will be over 19/16/14 years (whatever the true amount they paid is).
Chances are, it will increase by a lot more than 1 billion a year. There's just too much data they control.
If you want to spot how they might monetize on their data, look at FB for jobs about natural-language processing (now or in the past). Also look at their ad pages to see if they boast about targeted ads.