But 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. And by connecting your phone number with Facebook's systems, 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. You can learn more, including how to control the use of your data, here.
That's a big deal because a phone number is required to use WhatsApp, as its more than one billion users are already aware. Thankfully, current WhatsApp users who may be concerned over the prospect of their phone number being shared with Facebook have a way to opt out (via Venturebeat). When presented with the new, updated terms in the app, simply tap "Read" instead of agreeing right away. From there, you can scroll down to the bottom of the screen and uncheck the box giving your permission to share information with Facebook.
If you happen to accidentally agree to the new terms before reading, all is not lost: you have 30 days after accepting to change your mind. All you have to do is head to Settings > Account > Share my account info and uncheck the box.