When I create new app in facebook developer page, I noticed that my newly created app is using Graph API v2.0 instead of v1.0. I found this when I saw facebook, after authorization my app, giving me user_id of facebook user which was something different then the original user_id. Then I discovered facebook is hiding the actual user_id and give us app_scoped_user_id instead. This id is app specific meaning that same user have different app_scoped_user_id in different facebook app. So there is no way to get the actual facebook user id anymore.
Facebook released Graph API v2.0 which has lots of things to look at. These include facebook login, permission, dialog etc. Here is the upgrade guide from v1.0 to v2.0. Also look here for permissions for facebook login. Don’t forget to look at the best practice guideline for facebook login.