Also, some sites use a unified login which is shared between multiple sites owned by the same company. For example, StackOverflow uses an account which is share

Also, some sites use a unified login which is shared between multiple sites owned by the same company. For example, StackOverflow uses an account which is shared across all the various StackExchange-type sites, and Gamepedia requires you to sign in with your "Curse Community Account". (I'm sure there are more examples; those are just the ones off the top of my head.)