Lets make this clear from the start, I don't like Facebook, never have - never will. So before the "haters gunna hate" brigade start off here, the exit is that way.
Whatsapp, for those that don't know is a multi-platform messaging app that can send, text, pictures and audio / video as well, the app costs around 69p / $1 a year to use and is advert free. Pretty simple really. I have used it happily for many years and until today, thought I would continue to use it. The app install base is huge of course and millions of those messages flying about will have many uses to FB. Firstly there is nothing stopping the new owners changing the Terms and Conditions to include all the items you share over the app becoming the IP of Facebook, sounds familiar ? Sure it does, so sending anything which could be a little personal, be it data or some flesh could spell out a bit of a disaster waiting to happen.
Secondly the question of that 69p yearly sub ? Will it become replaced with adverts, based on the information it reads from your messages no doubt. Sounds a bit Google Search like but will I be able to see that data and remove it at will, like I can with Google ? I doubt that too, even if I could I suspect it would be kept for years on end just like the main business line of social ripping Facebook.com when you "remove" yourself from that site.
So the classic customer who likes to pay for something is going to lose out by surrendering their info in exchange for something free, yes everyone else is doing it these days and I personally think that trend could be changed - I'd much rather have a "paid for" service over free any day - but the internet isn't like that so much these days. Imagine if EVERYTHING was like that - "taxi please, sure I'll need all of your likes, personal pictures, friends names, phone numbers, birthdays, locations you have been oh and emails - hop in - Tescos was it ?" I'm not sure a single person would give up that information. What information Facebook will rip from your messages - nobody knows but the Facebook Messenger app is pretty intrusive compared to Whatsapp and merging the two would make sense for them to reap the benefits of Whatsapp huge database.
There is another reason why I think this purchase is going through, and that's a defensive play against Google - there is/was a rumor going round that Google bid for Whatsapp but regardless if they did or did not, Facebook letting Google purchase Whatsapp would have been a bit of a disaster for them. The reason being that Google would have merged it into Google+ quickly, increasing its ever growing user base, regardless of what everyone says about Google+ it is growing, and of course would have become free with G+ / Hangouts, which in my opinion is much better than most other message apps on the market anyway. No change to the Terms which read your SMSs would have been required either.
In some ways what Facebook are trying to do is stop the rot that's started to creep in, its becoming uncool, unsafe and most of all unsocial, purchasing an app with a massive user base giving it a new way of getting users data is great for them, but a social disaster for us - the end user.