Facebook has undermined privacy on its network by exposing mobile phone numbers provided to secure user accounts through two-factor authentication. That's because anyone can use the numbers to look up a user's account. One doesn't even have to be a Facebook member to do so. Moreover, there's no way to opt out of the setting, although it can be limited to "friends" only. The security gaffe came to light when Jeremy Burge, a UK entrepreneur, posted a tweet that drew an outpouring of responses from angry Facebook users.