Drupal 7 & Facebook Connect - Connect Button Problem! Help please?
I have the same issue, FB Connect will do this when your site tries to connect to an  FB App and cannot. Check your API and Secret keys first, other possible problems are the URL settings which should match in both FB and Drupal/FBConnect. Lastly in my case, everything was working until I reset an auth key in FB which I should not have done and it created issues for me.