This will help Facebook with advertisement targeting, but it will also give the social network more ways to track you around the web. This also means that Facebook will have info on those who don't have an account, but possibly use WhatsApp. Two important bits of information WhatsApp will be sharing include your phone number, along with the last time you used the service.
“By coordinating more with Facebook, we’ll be able to do things like track basic metrics about how often people use our services and better fight spam on WhatsApp. Facebook can offer better friend suggestions and show you more relevant ads if you have an account with them. For example, you might see an ad from a company you already work with, rather than one from someone you’ve never heard of.”
WhatsApp data won't just be going to Facebook though, it will be going to the ‘Facebook family' of companies, which would also include Instagram and even Oculus. Users will be able to opt out of some of the data sharing, so if you don't want targeted ads, you can put a stop to that, but you can't opt out of all of it.