Facebook has recently become in trouble, the company was found to use the user-submitted two-factor authentication mobile phone number to match the user to the real-life network of friends. Obviously, this news caused a stir, but it seems that Facebook always gets the user’s phone number, probably requesting it from the carrier, presumably via an API in the phone app. A new screen discovered by reverse engineer Jane Manchun Wong clearly illustrates this.
Facebook added this screen for whether to keep sharing user's phone number from mobile network to Facebook pic.twitter.com/jYmrRdTBKA
— Jane Manchun Wong (@wongmjane) September 14, 2019