Such a waste. Most Whatsapp users already use Facebook in parallel. Facebook will never see a ROI on this acquisition. Not even close.
Sometimes the ROI is in retaining your value and not dying, as opposed to getting $$ out of the investment.
I'm guessing if you want to be the #1 social network for a long time, you don't want people being social outside your platform. I'd put this purchase in the defensive bucket.