In my country WhatsApp is used for everything from talking to friends through setting up a date with your hairdresser to group activities like school parents groups.
There is an expectation that the information you share by someone having your number is very limited - the person that has your number can text you, yes, but they can't know about you, and you can limit the small amount of info you let through like your profile picture or your online state using privacy controls.
This expectation is completely removed when adding somebody's number to your contact list is enough for Facebook to do its magic and reveal the owner in your Facebook friends suggestions.
I've had it happen dozens of times, I start texting a tinder match and suddenly her profile is there in my suggestions. It's common for it to misfire and I end up being suggested the personal account of the owner of a business I bought something for. They don't even need to text you, you add the number to your phone's contact list and it's there.
Facebook needs to be broken apart, and we need a law that the data you share with an app can't be used for others period, even more so if they were separate businesses when you started using the service, and a change of policies is not enough - you might already be locked in.