If you already have an active session or an access token, you can do the following to get the user_id:
// set session from cookie or via helper
$session = new FacebookSession( $session->getToken() );
$user_id = $session->getSessionInfo()->asArray()['user_id']
echo $user_id
See this tutorial for information on using a session saved in a cookie or creating a new session. It's a long solution but this the easiest way to retrieve the user_id using the new SDK. It's advisable to save it in a session for easier retrieval.