Session still exist after facebook logout in my application

hi i am using facebook graph api for invite friends in my codeigniter application . if i have already logged in to facebook the following screeen is shown

if i am not logged in the page redirects to facebook login page , after suceesfuliy login redirects back to above page . every thing works fine . but the problem is

1) i log in to facebook .  
2) i run my application and get the list of friends to invite (above view) . 
3)then i log out from facebook . 
4)then refresh (F5) my friends invite page .

exeption occurs ,

part of the exception

Error:FacebookApiException Object
(
    [result:protected] => Array
        (
            [error] => Array
                (
                    [message] => Error validating access token: The session is invalid because the user logged out.
                    [type] => OAuthException
                )

        )

    [message:protected] => Error validating access token: The session is invalid because the user logged out.
    [string:Exception:private] => 
    [code:protected] => 0
    [file:protected] => C:\xampp\htdocs\elephanti\application\frontend\libraries\connections\facebook.php
    [line:protected] => 530
    [trace:Exception:private] => Array

my code is

public function invite_friends()
{

    $fbconfig = array(
        'appId' => $this->co_config_items['fb_appid'],
        'secret' => $this->co_config_items['fb_secret'],
        'cookie' => true,
        'domain' => $this->ci->config->item('domain')
    );

    $this->ci->load->library('connections/facebook', $fbconfig);

    $facebook = new Facebook($fbconfig);

    $session = $facebook->getSession();    

    if (!isset($session)) {

        $url = $facebook->getLoginUrl(array(
                    'canvas' => 1,
                    'fbconnect' => 0
                ));

        echo "<script type='text/javascript'>top.location.href = '$url';</script>";

    } else {

        try {

            $uid = $facebook->getUser();  
            $me = $facebook->api('/me');

            $updated = date("l, F j, Y", strtotime($me['updated_time']));

        } catch (FacebookApiException $e) {

            echo "Error:" . print_r($e, true);
        }
    }
    $data=array('fb_appid'=>$this->co_config_items['fb_appid']);

    return $data;
}

exception prints from the

} catch (FacebookApiException $e) {

    echo "Error:" . print_r($e, true);
}

block

the thing is in the if else condition the else part is working , but there shouldn't be a session when i logged out from the facebook . but the session has not cleared , i var_dump the sessions in the first and second time .

before log out

array(7) {
  ["session_key"]=>
  string(46) "2.AQC0otQQWydT6w0l.3600.1317193200.0-770398599"
  ["uid"]=>
  string(9) "770398599"
  ["expires"]=>
  int(1317193200)
  ["secret"]=>
  string(24) "E9oNYQCuQdggyp_pJyLnqA__"
  ["base_domain"]=>
  string(9) "localhost"
  ["acces开发者_高级运维s_token"]=>
  string(90) "140577549320080|2.AQC0otQQWydT6w0l.3600.1317193200.0-770398599|5ZoDt4wTt16ve0fgaj8m_X70ojw"
  ["sig"]=>
  string(32) "74a283a7cd2f4d62bb7739deaf65affc"
}

after logout and refresh one time

array(7) {
  ["access_token"]=>
  string(90) "140577549320080|2.AQC0otQQWydT6w0l.3600.1317193200.1-770398599|C84xhtx_dnakmf-7oCZlb07FQL8"
  ["base_domain"]=>
  string(9) "localhost"
  ["expires"]=>
  string(10) "1317193200"
  ["secret"]=>
  string(24) "E9oNYQCuQdggyp_pJyLnqA__"
  ["session_key"]=>
  string(46) "2.AQC0otQQWydT6w0l.3600.1317193200.1-770398599"
  ["sig"]=>
  string(32) "6eedc49e4957708a551c4155ff8f57a6"
  ["uid"]=>
  string(9) "770398599"
}

in the second refresh the session is NULL and redirects to the login page and works correct , why is session not clearing in the first time , please help .

UPDATE

after session is NULL i print the session and addede a die , and logged in to facebook again ;

$session = $facebook->getSession();

print_r($session); die ;

the thing is every time now gives NULL , so i think a place after

 $session = $facebook->getSession();    

session is creating , i think that's why i get the correct result in the second time , from where the session is changing ??

i searched and saw similar questions but could not get help. please help ..............

Try adding a listener for when the user logs out;

FB.subscribe('auth.logout', function(response){
    // Do something to let your program know that the user is now logged out.
});

And/or, add your own facebook logout to your site with FB.logout(). This should clear your website/s fb session as well as log them out of facebook.com.