Actually this is overly complex and I have added to the confusion.
There is a difference between say, Facebook or twitter account linking, and logging in.
You need to have only one login authority per user. The other type of linking is extra and not related to login.
user_id refers to a local user in OpenACS. The external authority_id just says which authority to login with. There is ever only ONE username/password combination that will login a particular user_id.
