Perhaps you've seen this terrifying warning (or some permutation thereof) pop up on your Facebook news feed lately:
ALL OF THE PHONE NUMBERS of your contacts are now visible on Facebook. go to the top right of the screen, click on ACCOUNT, click on EDIT FRIENDS, left side of screen and click CONTACTS. you will see all phone numbers from your phone (FB friends or not) are published that you have stored in your mobile phone. TO REMOVE, go to the right column, click on "this page."
Did that seem fishy to you? The sort of warning your septuagenarian aunt would give you? Perhaps flat-out illegal, and the kind of thing no well known company (barring NewsCorp, perhaps) would try to get away with? The sort of thing that a social media site couldn't do until it almost literally controlled the world? You'd be right: Facebook is not hacking into your phone and copying down people's phone numbers. I'll let the smart people over at the Times explain it to you:
If you recently installed the Facebook mobile app onto your smartphone, you had the option to sync your phone contacts with Facebook. For most people, the main payoff was that friends’ Facebook profile pictures would appear onscreen when they called.
But what you were doing was allowing Facebook to keep tabs on your phone’s contact list (you got a pop-up box basically telling you that). That’s how Facebook is able to determine that the Sam Grobart in your phone is the same Sam Grobart you are friends with on Facebook.
So Facebook has the content of our phone’s contact list. And that’s because we let it.
At the same time, Facebook users have the option to display their phone number on their Facebook profile page. But you can restrict who sees your phone number, including the option that no one can see your phone number.
So, yes: Facebook knows your phone number, but only if you let it. And while there are many legitimate privacy concerns when it comes to the social networking site — like giving your information to corporations, to name just one — this is more along the lines of someone finding that picture of you doing body shots off of a half-naked bar attendant. Your future boss might see it, but that's why you don't put it online.