I know, it's been a long discussion already about how to use both, wordpress comments and the facebook comment box, but none of the solutions worked for me so far, until i found this thread: http://wordpress.org/support/topic/facebook-and-wordpress-comments?replies=9
the code has changed in 1.1.10 so I had to adapt the dirty fix. ;)
uncommenting line 256 (in facebook.php in the facebook pluginfolder)
// add_filter( 'comments_array', array( 'Facebook_Comments', 'comments_array_filter' ), 10, 2 );
and line 310 (in facebook.php too)
// add_action( 'wp_enqueue_scripts', array( 'Facebook_Comments', 'css_hide_comments' ), 0 );
now does the trick.