Motivation
An email address will be the singularity that the Accounts table will be founded upon. An account is the starting point for any identity. An account may be entered into from multiple login providers (username/pass, facebook, google, github, etc). An account will act as the entry point for a grouping of customers/users/characters. After logging in, a user will be given the option to select a user-entity or a default user-entity may be automatically selected. Single-user systems will be a commonplace structure. It may be easiest to think of an account in terms of a single billing source that may contain multiple user-entities.
The proposed schema can be represented in either a SQL or NoSql database. I have only included a bare minimum of fields and expect this to grow as requirements are identified.