How to design a schema for a site that accepts logins from multiple sites (e.g. facebook connect, google, flickr, etc..)?
There is a similar question in stackoverflow

And the solution is


  You need 2 additional fields in your
  table, one is the login source
  (twitter, openID, facebook) and the
  other is the source user id (the id
  provided from twitter, openID,
  facebook) I recommend making this a
  string as some log in sources have
  letters in their user IDs and not just
  numbers. more



I'm not sure how smart it is to use this approach.  



  This approach is good, actually sites
  opting it. see SO login page
  https://stackoverflow.com/users/login



I don't know how to implement the third scenario in such an approach.  



  I think  you can implement this, only
  if the thrid party sites provide
  access. This question is off-topic
  here, you can ask this in
  stackoverflow