yea VB / PHPBB they have Session Fixation Implemented already, this mainly for personal apps you create.

Another method that i would bet face-book's code goes along is something like this .

PHP Code: 
$timeout = 60 * 60; // 1 hour
$fingerprint = md5('MY-SECRET-SALT'.$_SERVER['HTTP_USER_AGENT']);

session_start();

if(
    (isset($_SESSION['last_active']) && (time() > ($_SESSION['last_active']+$timeout)))
    || (isset($_SESSION['fingerprint']) && $_SESSION['fingerprint']!=$fingerprint)
    || isset($_GET['logout']) )
{
    //Logout!
}

session_regenerate_id(); //ALWAYS BEFORE
$_SESSION['last_active'] = time();
$_SESSION['fingerprint'] = $fingerprint; 
And within your html create a javascript file to ping the server every 30 seconds to keep them alive. this is called a heartbeat.