If you think that was their intention with this acquisition, you're mistaken. WhatsApp poses a threat to traditional social networking, and Facebook obviously wants a leg in that race.
The Facebook app can already:
-read your SMS messages
-modify/read your contacts
-read/write your call log
-read the contents of your storage
-retrieve running apps (i.e. it knows every app you have installed on your device, given enough time)
There's so much talk about how Google knows its users better than anyone else, but I think there's little doubt that Facebook gets the throne as far as data collection goes. And lets not forget that in addition to all the above data Facebook knows the 99% of your browsing activity and even when you opened a certain website.
There's really nothing more they can take from us, so I highly doubt this acquisition was meant to bolster their data collection capabilites.