Question

can I retrieve it using fql or graph api

Answer 1

Each time a user comes to your app, his facebook id can be retrieved like this:

$facebook = new Facebook($api_key, $secret);
$facebook->require_frame();
$user_id = $facebook->require_login();
echo $user_id;

Now the facebook id is stored in the variable $user_id. Once you receive an id like that, you should save it in your database and you can get them back from your database any time you like 🙂