Two-factor authentication (2FA) is one of the primary ways you stop your online accounts from being hijacked by criminals who manage to guess, steal, or work out your passwords. The idea is that a hacker might know your password but they dont know the six digit security code but even if they did find it out they would be hard pressed to make use of it. Facebook let anyone look up the owner of that particular phone number just by entering it. Despite that phone number only being given for the purposes of enhancing security, Facebook already exposed for doing this.”]
Source: https://grahamcluley.com/facebook-2fa-phone-number-look-up/