How do you design user authentication for an app that runs on the open web and social networks?

  CREATE TABLE User (
         id serial PRIMARY KEY,
         full_name varchar(100) NOT NULL DEFAULT 'Anonynous Coward',
         ...
  );
  
  CREATE TABLE FacebookUser (
         fb_id int UNIQUE NOT NULL
  ) INHERITS (User);
  
  CREATE TABLE AppUser (
         uid int UNIQUE NOT NULL
  ) INHERITS (User);
  
  
  CREATE TABLE Friend (
         uid integer REFERENCES User (id),
         friends_with integer REFERENCES User (id)
  );