tampoco en ELGG 1.8.8

I modified the start.php and test it online, it works well.

From "register_elgg_event_handler" to elgg_register_event_handler;

 elgg_register_event_handler('init','system','fb_commentbox_init');

BTW, i also change the width to fulid width. you can add one line in the plugin files:

 elgg_extend_view('css', 'fb_commentapps/css');

and then create a css.php under the fb_commentapps folder, then add the code below into the css.php:

<?php

?>

.fb-comments,.fb-comments span,.fb-comments iframe[style] {width: 100% !important;}

and then save it.

Enjoy:) 

As suggested by Weipeng it works for me too.