CodeIgniter中的facebookconnectphp-sdk问题

我的facebookconnectphp库有问题。基本上在返回url时，我的用户是0。这是我的代码：

public function facebook_login()
{
    $params = array(
        'appId' => 'XXXX',
        'secret' => 'XXXX',
        'cookie' => TRUE
    );
    $facebook = $this->load->library('facebook', $params);
    $user = $facebook->getUser();
    if (empty($user))
    {
        $url = $facebook->getLoginUrl(array(
            "redirect_uri" => site_url('/auth/facebook/connect') 
                ));
        header("Location: $url"); 
        exit; 
    }
}
public function facebook_connect()
{
    $params = array(
        'appId' => 'XXXX',
        'secret' => 'XXXX',
        'cookie' => TRUE
    );
    $facebook = $this->load->library('facebook', $params);
    $user = $facebook->getUser();
    print_r($user);die;
}

$user在返回url上始终为0。我该怎么解决这个问题？