This is what you want the T&Cs screen to look like *before* you tap ‘Agree’.
Once you’ve switched the toggle off you’re free to tap Agree — safe in the knowledge you have at least minimized how Facebook can use your WhatsApp data.
It won’t be able to use your WhatsApp data for ad-targeting, or for trying to triangulate the identities of people you know on WhatsApp with Facebook profiles to push more friend recommendations your way.
It will still be able to use your WhatsApp data for some uses though, including “improving infrastructure and delivery systems, understanding how our services or theirs are used, securing systems, and fighting spam, abuse, or infringement activities”, as WhatsApp tells it.