Identity and email address
Users accounts have as unique identifier in the system an email address. Other authentication mechanisms such as Facebook, Google, LinkedIn or Twitter logins are possible, however all these "identities" are associated to a user account defined by an email addess.
A relatively common "problem situation" is that an user already has an account on the system using, let's say, the email address given by their internet provider, i.e. like firstname.lastname@example.org. The user also has a Facebook account, but in Facebook their email address is a email@example.com. When the user logins in the system for the first time using the Facebook authentication, an user account is automatically created with the address firstname.lastname@example.org. This situation can be solved only be technical support, who will associate the newly created identity based on Facebook/LinkedIn/Google, etc. to the existing account