Letztlich lässt sich alles irgendwie kopieren oder missbrauchen. Kein System ist perfekt.
Facebook erkennt die Zeichen der Zeit: http://www.theverge.com/2016/4/12/11411296/facebook-accounts-phone-number-log-in-f8-conference-2016
"Accounts, which were introduced on stage at today's F8 developer conference, represent a significant expansion of the familiar "log in with Facebook" button. Apps that enable logins with Accounts ask you for your phone number. Once you send it, Facebook sends you a code to confirm the number belongs to you. Enter the code and you're now logged into the app, with no password to remember. "People tell us that they hate passwords," says Eddie O'Neil, a product manager at Facebook."