I've read the instructions, but in your tutorial you first have to upload. In the Heroku instructions you first download (git clone), change (edit *.php) and upload (git push).
So what should come first?
I've deleted alle the files in the map created by git clone, except the file .gitmodules. Next placed the files from facebook_server_auth_app into the map created by git clone. Finally used git push to upload again.
Got no errors, but the facebook app hasn't changed.