I was getting an error on the product detail page that the domain used in a call to facebook's API wasn't in the list of approved domains. Upon checking the code, I note that it attempts to get the value of "app_id" from the configuration. This variable is not defined anywhere, so it uses a default value which is not mine, of course.
I had to go into helper.php and change the default value to get it to work.
Where is app_id defined, so I can properly set it, and not clobber my change to helper.php next time there's an update?