How to relate the user table with the fboauth table to show up who's logged in through fb?
You have two options.


First is that you can add a field to the USER, named as fb_user(Keep the default value as "no"). Now look into the fbouth code where a user is being created. Write a sql query there using Database API to update the field fb_user as "yes". Now you can easily add the field in views to find out FB users.
Create a custom module. Inside that write a hooks_views_query_alter:-https://api.drupal.org/api/views/views.api.php/function/hook_views_query_alter/7
Custom query in Views?