Not entirely true.
If you install the app on your smartphone, as soon as you sign in to FB with it, the app uploads all the contact info from your phone book.
All the contact info.
The phone numbers of your mistresses on your phone? FB knows you know them.
Your siblings who refuse to be on FB? If you have their contact info in your phonebook on that phone, well FB now know about them and that you are somehow connected to them.
It's naive to think social media only has what you post on it and nothing else.
It has more on you than you care to know.
The Sync functionality has to be enabled. It’s in the settings of your phone under the Facebook setting. Facebook needs permission to access anything on your phone.