The whole WhatsApp acquisition touches on some of the same personal nerves that Google's acquisition of EtherPad did. It was a really great product that I used daily. At the time Google had big plans for their Wave product, and the entire deal seemed to make sense. Fast forward a few months and Wave is a dud and EtherPad is no more.
I'm not saying that WhatsApp won't be useful to Facebook, but I do think that the philosophy and goal of the two companies are very different. Ads is one potential "technology" that could monetise the product and it's obviously against the founders' philosophy. There will however probably be many other (more subtle) influences from Facebook's side that'll slowly shift it away from the original vision - and promises to users. That's life I guess: take the cash or keep you vision.
The project lost a lot of momentum after Google acquired the developers working on Etherpad. I think the original codebase got forked as the Etherpad Lite project that is still being developed.
I'm not saying that WhatsApp won't be useful to Facebook, but I do think that the philosophy and goal of the two companies are very different. Ads is one potential "technology" that could monetise the product and it's obviously against the founders' philosophy. There will however probably be many other (more subtle) influences from Facebook's side that'll slowly shift it away from the original vision - and promises to users. That's life I guess: take the cash or keep you vision.