The messages you send are encrypted, WA/FB can't see the content you send (so they say, but that's another discussion). You may or may not have a FB account or link to any other social media. WA/FB say they want to use your number "for advertisements or product-improvement purposes". How they are going to do that one can only speculate. It most likely has to do with who you connect with rather than what you exchange as this is the info they have from your phone number and those you connect with through WA. Add in any of your contacts that also have accounts on FB or are connected to FB, and you can start putting the bits together.
If you have spent $16Billion on buying an app that gives you access to 1Billion users, there is no way that you are not going to find a way to recoup that. For years users have requested features (like blocking being added to groups) only to be ignored and have bloat added to the app (voice and now video calling).
I found that WA was becoming a pain in the rear. I ditched it for Telegram and SMS. Those I want to connect with are available. The endless steam of "thumbs up" and "thanks" that accompanied every single group post became too much for me. 2 months of pure pleasure so far.
