LinkedIn and Facebook can do this because your email is not your username.  There likely isn&#x27;t really a username in those systems, just a user id.  Almost all systems have a user id, but the distinction here is slightly different.  Instead of a user record which has an id, username and possibly full name, Facebook likely has a user record with an id, a (display) username, and then there is a separate set of auth records with the multiple ways you can authenticate to the account, such as emails and passwords, API tokens, phone numbers, etc.<p>It may not sound like a big distinction, but there <i>is</i> a big distinction there.  Instead of your username being your email address, they&#x27;ve abstracted the authentication from the core user record such that usernames are not used for authentication, so don&#x27;t have to mutate if the authentication identifier (email address, phone number) changes, just some auth settings.  This is obviously a much more extensible and robust way to deal with authentication over time, but it&#x27;s also obvious it&#x27;s much more complex than a simple username&#x2F;password pair.