Facebook's 2FA 'Security' Practices Violate User PrivacyFacebook 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.
Source:
Facebook's 2FA 'Security' Practices Violate User Privacy