automatically post a feed after login

Here is the Solution:

$feed = get_my_feed();
publish_my_message($facebook, $feed);

/*
* Return array describes how the login window will look like
*/
function get_my_feed() {
	global $base_root;
	
	$msg = t('My Message!');
    $attachment = array(
         'name' => t('My Site Name'),
         'href' => url( $base_root, array('absolute' => TRUE)),
         'description' => t('The description of the attachment'),
         'media' => array (array(
                'type' => 'image',
                'src' => 'path/to/logo.jpg', 
                'href' => 'link/to/somwhere',
         )),
     );
	
	return array('message' => $msg, 
	             'attachment' => json_encode($attachment));
}

/*
* Return the user facebook info
*/
function get_me_info($facebook) {
	$me = null;
    $session = $facebook->getSession();
    // Session based API call.
    if ($session) {
        try {
            //drupal_set_message("Got Session");
            $uid = $facebook->getUser();
            $me = $facebook->api('/me');
        } catch (FacebookApiException $e) {
            error_log($e);
        }
    }
    else {
        //drupal_set_message("No Session");
    }
	
    return $me;
}

/*
* Publish the message on the wall
*/
function publish_my_message($facebook, $feed) {
	$session = $facebook->getSession();
	$me = $facebook->api('/me');
	publish_stream($facebook, $session, $feed);
	return $me;
}

/*
* a wrapper for the stream.publish facebook method
*/
function publish_stream($facebook, $session, $feed) {
	$userid = $facebook->getUser();
	$access_token = $session['access_token'];
	$facebook->api(array('method'=>'stream.publish',
	                     'message'=> $feed['message'],
	                     'attachment' => $feed['attachment'],
	                     'target_id'=>$userid, 
	                     'uid'=>$userid,
	                     'access_token'=>$access_token)
	);
}