In today's cutting edge world of technology, there have been ongoing battles about privacy online, and privacy features. Facebook is no exception to this, as they have had numerous privacy related concerns. Now users will be struck with paranoia once more by a "feature" in Facebook's iPhone and Android apps.
Facebook Sync is to blame for this apparent privacy blunder. The service attempts to sync your friends' profile pictures with the contacts in your phone. To do this, Facebook will upload all of the numbers in the address book and essentially do a quick comparison, according to The Guardian. The information gets uploaded to Facebook's Phonebook app, which is essentially a place where you can find all of the numbers of your contacts on Facebook. That's fine if you're happy with that but the warning was not clearly present on previous versions of the Facebook iPhone app. If you enabled this functionality previously then you may want to check this page to see if any phone numbers are listed.
You may be surprised at the plethora of numbers found. Some of the users added themselves, however if there were any blanks, Facebook Sync will fill these in. This isn't even just for your friends only, this applies to everyone in your address book. You are shown a warning (shown above), but still if you agree, which most people will, then privacy goes right out the window. What's not clear at the moment is how the algorithm works out and shares your numbers and if it's sharing your numbers to others.
Perhaps the more scary part to all of this is that via the Facebook API, all of this information can be easily accessed. Tom Scott's EVIL page demonstrates this incredibly well. While he anonymised the phone numbers, Scott still has shown just how easy it is to exploit this.
Image source: guardian.co.uk