Misconfigured application or install. Please see the "Help" section and review the configuration steps very carefully. You can not configure your site for example.come and then attempt to use FBC on
www.example.com
, the 2 sites are not the same. It could also be a conflict with 1 of the 2 other Facebook based extensions you've installed as the APIs fight for control; try disabling them and see if issue persists.