Facebook requires your number and then links your data

I don't use Facebook anymore, but I had to just sign up so I could test the new Facebook authentication within an app I'm working on. I naturally used an alias email address and a fake name (Boberius Kingslayer). Upon registering I needed to enter my phone number due to 'suspicious activity'. Okay, fair enough - I need to test this so I enter my real number which I've had for a few weeks under the assumption it just required an activation code and wouldn't link to my account.

I'm then shown a list of suggested friends. This included my sister, neighbor, friends from school... essentially the top 50 people suggested were spot on. The only app linked to my phone that this could be related to is Whatsapp, however I just read the T&Cs of Whatsapp/Facebook integration and they specifically say they don't share contact details: "Importantly, WhatsApp does not share your WhatsApp contacts with Facebook or any other members of the Facebook Companies, and there are no plans to do so."

I can't even remove my mobile number from Facebook without it threatening to delete my account.

I signed up via a desktop browser and didn't share my contacts.

I'd be interested if someone could explain how this can happen without breaking their own T&Cs somewhere?