First, it is very likely that the WhatsApp encryption is compromised, it definitely shouldn't be trusted, as it is completely proprietary and thus not transparent to users and independent auditors. Also, unlike Signal, WhatsApp doesn't encrypt any metadata. The biggest source of WhatsApp user data for Facebook though are address books. When you grant WhatsApp permissions to access your contacts, that data is sent to Facebook servers unencrypted. That way, Facebook can see the names and phone numbers of all of your contacts. This is not just bad for you, it's also bad for everyone whose phone number you saved in your address book, their data is sent to Facebook, even if they don't use any Facebook services themselves. Also, when you have WhatsApp or any app installed on your phone, it by default has access to many things that you can't control or restrict. For example, it can access some unique device identifiers and look at stuff like the list of apps you have installed on your phone or access sensors like the gyroscope and accelerometer which can absolutely be used to track you. It's better to keep shady apps like those made by Facebook, Google, Amazon, Microsoft or other surveillance corporations off your devices. Use FOSS alternatives with a proven track record like Signal if they are available.