Publish a message on facebook after having posted a comment

I would like some advice on how to improve the code I use for allowing people to comment posts.

Users can only submit a comment if they are connected with Facebook.
I started developping this feature directly in the comments.php template. It works but there is another feature which permit people to publish a message on their wall if they tick a checkbox before sending the comment.

I have some troubles for developping this functionnality because I need an instance of the Facebook class. I already have it in my comments.php and I was wondering if there was a way to use it instead of creating a new one.
Here is the code :

comments.php:

<?php 
locate_template( 'include/facebook.php', true );

$config = array();
$config['appId'] = get_option( 'appId' );
$config['secret'] = get_option( 'secret' );

$facebook = new Facebook($config);
$session = $facebook->getUser();
if(!$session){
    $loginUrl = $facebook->getLoginUrl(array('scope'=>'publish_stream, email', 'redirect_uri'=>'http://www.myredirecturi.fr'));
}

if( !isset( $session ) || empty( $session ) ){    
    ?>
    <div class="btns">
        <div class="fb-connect">
            <a href="https://wordpress.stackexchange.com/questions/113481/<?php echo $loginUrl; ?>"><img src="<?php echo get_template_directory_uri(); ?>/img/btn-fb.png" alt=""></a>
        </div>
    </div>
    <?php
}

$nb_coms = get_comments_number();
if( $nb_coms != 0 ){
    wp_list_comments('callback=custom_comment&avatar_size=50');
}
?>

<?php
if($session){    
    try {    
        $me = $facebook->api('/me');
        $username = $me['username'];
        $firstname = $me['first_name'];
        $email = $me['email'];

        // I display here my custom comment form        
    }    
    catch(FacebookApiException $e){    
        error_log($e);    
    }
}
?>

functions.php

<?php
add_action( 'comment_post', 'custom_save_comment_data' );  
function custom_save_comment_data( $comment_id ) {  

    add_comment_meta( $comment_id, 'fb_username', $_POST['fb_username'] );

    //if( isset($_POST['publish']) ){
    //  $params = array(
    //                  'message'       =>  "Hurray! This works :)",
    //                  'name'          =>  "This is my title",
    //                  'caption'       =>  "My Caption",
    //                  'description'   =>  "Some Description...",
    //                  'link'          =>  "http://stackoverflow.com",
    //                  'picture'       =>  "http://i.imgur.com/VUBz8.png",
    //              );
    //  $user = $_POST['fb_userid'];
    //    $post = $facebook->api("/$user/feed","POST",$params);
    //}
}
?>

What do you recommend for pass the facebook object ? Does the way I developped these features is correct ? or should I create a plugin ? if it’s the case, why ?

or should I create a plugin ? if it’s the case, why ?

Yes you should: Separation of concerns. A theme is the view of the application. As the facebook functionality is part of your application, it belongs into a plugin.

You have one large benefit from moving stuff into plugins: If you exchange your theme, you can easily switch themes without loosing the functionality and having to code all that again.

And always use hooks and (even better) filters to bring stuff into templates. Hooks like the_content, start_loop, end_loop and the_post are perfect for attaching plugin output to the view. And they’re present per default as core injects them into loops.