Facebook recently landed in hot water after it was discovered that they used mobile numbers users submitted for two-factor authentication to match users to their real-life friend network using the phone numbers from those friend’s phone books.
This means for example that Facebook would be able to match you up with your plumber, drug dealer or a one-night stand, simply because they saved your phone number in their phone book and shared their phone book with Facebook, despite you never intended for Facebook to have this information about you.
That revelation caused a ruckus, but it seems Facebook always had the ability to get to your phone number, merely by requesting it from the carrier, presumably via an API in the phone app.
A new screen uncovered by reverse engineer Jane Manching Wong made this fact clear and made users aware that they had the ability to opt-out of this automatic data scoop.
While some may say your phone number is not exactly private, given that for most of us it is available for free in the phone book, it is also true that for a phone user it remains a unique identifying feature, and a lot of information is revealed by who has and uses the number.
Facebook is currently under antitrust investigation in USA, and hopefully, some robust action could curtail some of the worst excesses of the company.