I succeded to fetch Facebok comments number via this function:

<?php 
function fb_comment_count() { 
global $post; 
$url = get_permalink($post->ID); 

$filecontent = file_get_contents('https://graph.facebook.com/?ids=' . $url); 
$json = json_decode($filecontent); 
$count = $json->$url->comments; 
if ($count == 0 || !isset($count)) { 
    $count = 0; 
} 
echo $count; 
} 
;?>

And I call it with:

<?php fb_comment_count();?>

Now how do I add it to this code:

<?php comments_number(__('No Comments'), __('1 Comment'), __('% Comments'), '', __('Comments Closed') ); ?>

so that WordPress shows number of WP and FB comments together in one number.

Thank you very much to everyone!