Why you should delete your old phone number on Facebook: Simple profile recovery trick that relies on outdated details lets anyone break into your account.
James Martindale, a security expert based in Colorado has exposed a flaw in Facebook’s account recovery system that can allow a hacker to easily access your Facebook account using a simple trick. Facebook's account recovery system lets users log in using their phone number if they lose their password, but if a hacker buys an old phone number that is still linked to a Facebook account, they can easily gain access to the profile through the account recovery system. If a user forgets their password they can request a code to recover their account. But if someone has a new phone number and has not unlinked their old one from their Facebook account, hackers can type in the old number into the account recovery screen to gain access to the profile, change the password and access the Facebook account.