FaceBook Setup Issue

First use this url to ask user to add your facebook app(if user not login, facebook will redirect to login page and then go to this url)
https://graph.facebook.com/oauth/authorize?redirect_uri=< http://YOUR_FaceBookCallback_SERVLET>&client_id=<YOUR_app_key>

Then you can use this url to ask user grant the extend permission(allow offline access)
http://www.facebook.com/authorize.php?api_key=<YOUR_app_key>&v=1.0&ext_perm=offline_access&next=< http://YOUR_FaceBookCallback_SERVLET>=true&return_session=true

Then you will have an auth_token in the url, use Get to take it out and use it to generate the infinite session key. For php version, it looks like this:
1. $auth_token=$_GET['auth_token'];
2. $infinite_key_array = $facebook->api_client->auth_getSession($auth_token);
3. print_r($infinite_key_array); if the key is infinite, the expire will be 0

The result will be: Array ( [session_key] => c03eed6d6f07f335d4a36cd2-598500288 [uid] => 598500288 [expires] => 0 )
c03eed6d6f07f335d4a36cd2 is the key , while 598500288 is the user id

Thus you can store the infinite key for future use

Optional reading:

1.This mention Facebook is going to stop granting infinite sessions to Web-based applications automatically,
Desktop applications will continue to be granted a 24 hour session key, and mobile clients will continue to receive infinite sessions :
http://wiki.developers.facebook.com/index.php/New_Design_Platform_Changes#Changes_to_Session_Keys