One year ago the deal would have costed Facebook about one billion US dollars. So the wait proved costly. But at that point WhatsApp was not as widely known as it is now.
The ecosystem of social media platforms and applications is very dynamic. Not all of them survive the forces of the market and popularity. Not all of them promote good practices. Just remember Facebook Beacon's application. It would connect Facebook with your purchases and would let everybody on your network know what you had just bought.
The approach in that case was different. It was the application collecting data without your full consent. Now it is you who give that informants to Facebook via another application called Foursquare. So you want to be popular and tell everybody you are in an expensive restaurant. So you login in Foursquare. And then Facebook (with your consent, presumably) would make sure advertisers know about your preferences. The same can happen with WhatsApp users...
Facebook has grown wiser. It is now more careful, it waits for people to adopt a new application before buying it or implementing a similar one. But those people developing applications are also wiser, and they know that they can position themselves as indispensable and ask for anything in terms of money. So that Facebook will have to take an interest later on when they have proved to be popular and acceptable.
Will this transaction be beneficial for both parties? Well, WhatsApp owners have got some cash and shares. Possibly they will also lead their company from the inside. Like a good football player that is bought by a good team, the future lies not only in the player but on how the team play together. We will see how well the management of this new venture does.
Facebook wants to be the only social platform that people use to connect to each other. The more we connect to Facebook the more they know about us, the more people like me look for alternatives. I think I have grown older, bit wiser and definitely more expensive :)
