You can find my thread almost 10 years years where I questioned the way Facebook accessed my yahoo contacts.
That is in fact their secret weapon as to how they grew so fast. They would download all your yahoo contact info claiming it was just to see if they are on FB already or to send them an invitation (I guess they used the same harvesting script as one of the first pop web apps the birthday calendar reminder and Hi5 app ) .
Pretty much everybody thought I was weird for caring about this.
Stealing your data and your friends data was baked into Facebook from Day 1