If they are ready to pay $ 16B for WhatsApp it means that it's worth, for them, $ 16B.
For me the take away is that Facebook struggles to grow in "underdeveloped" countries and will pay whatever it takes to make sure these zones are easy to conquer.
Those markets are the growth of tomorrow and Facebook is certainly plateauing at home.
In the long run, I think Facebook will become some sort of shell company for "anything social".