Such a waste. Most Whatsapp users already use Facebook in parallel. Facebook will never see a ROI on this acquisition. Not even close.
I wouldn't think of this from a ROI perspective. Facebook might be trying to go for the monopoly on mobile messaging. And once Facebook controls that, they can do anything.
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.