Drupal 7 - Anyway to make Facebook Picture take place of User Picture if not available
You can use the Views PHP module and use PHP to create whatever logic you want for this image output.


  This module allows the admin account (user 1) to add fields, filters
  and sorts to views which use PHP code. It also supports additional
  accounts when the core PHP Filter module is enabled.
  
  This is the successor of Views Custom Field for both D6 and D7. The D6
  version supports both Views 2 and Views 3.
  
  Note
  
  While this module allows you to directly use PHP inside views
  which may be useful for quick and easy solutions, it is highly
  advisable to use regular handlers and plugins when available (or even
  to create one yourself). Take note that filtering and sorting a view
  using PHP always has a considerable perfomance impact.


So in your view you can have:


3 fields, user profile picture, facebook picture and the php field used to display a picture.
for the user profile picture, make it a default image display and exclude it from the output.
for the facebook picture make it a default image display and exclude it from render output.
in the php field you simply want to check if a file exists for user profile picture and then drupal_render() or return the profile or facebook picture Views will have already formatted for you using the other 2 fields.
be sure the 2 image fields are higher up in the field list of the View than the PHP field, so the PHP field can make use of those values in code.


EDIT

An alternative that is slightly less magical and tied to Views is to use Display Suite and add a Custom Field in display suite for a User. This custom field does basically all the logic between the 2 fields and it's literally a pseudo-field for Users. Views can then use the view_mode for Users that you've provided by DS.