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.
The market has shown time and time again that in the messaging world it's extremely hard for one force to stay dominant for too long. Unless this pattern suddenly breaks today, Facebook will continue having to shell out many billions to keep buying these companies. I'm not convinced Facebook's profit is enough to keep that up.