Associating Nodes with Anonymous User
The user ID Drupal uses is contained in the uid field of the {users} table. On that table, the user ID for the anonymous users is 0.  

You cannot alter the user ID to be a different value, especially if that value is not even contained in the {users} table. If you alter the user ID associated with a node, you are changing the author of that node.

I don't think it is even possible to associate an anonymous user with a Facebook account. What for Drupal is an anonymous user is not really a single user but a multitude of users that access a Drupal site without logging in; also the Google bot is an anonymous user, and associating it with a Facebook account would not make sense.