Try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit includes/config.php and add this line right under <?php
PHP Code:
define('DISABLE_HOOKS', true);
If that doesn't fix it, then can I please get screen caps of your Settings page on the facebook developer apps site (Basic Settings and Advanced Settings) so I can see what you have entered there. And, can we get a screencap of your AdminCP > Settings > Options > Site Name/URL/etc page also.