I don't know the actual reasons, but people are looking at the question from the perspective of "what does Facebook get out of this" when the opposite might act

I don't know the actual reasons, but people are looking at the question from the perspective of "what does Facebook get out of this" when the opposite might actually be true. They may have seen that someone else could buy up WhatsApp and pose a real threat to Facebook's market share. So it might be entirely possible that this was a defensive buy. If spending $16 billion now meant that you didn't have to spend $50 billion later on it would make sense. One does not simply spend $16 billion without giving it a good deal of forethought why.