I have a native (FBML) Facebook Application. I do not want to push the application visitors through the authorization process, however I do want to know their Facebook ID.
Is it possible to find the visiting users Facebook ID without requiring them to "Authorize" my Application?
Before, I said it couldn't be done - but it can.
http://developers.facebook.com/docs/authentication/canvas
Read that, it's excellent. You need to decode the Base64 string and check the signature is correct, but other than that it gets you the user id among other stuff (like a temporary access token for graph.facebook.com)
REMEMBER : You need to change the settings in the Migrations tab for the information to come through! It's a vital step, but can be overlooked...
Hope this helps :)
Stuart
来源：https://stackoverflow.com/questions/2990754/visiting-users-facebook-id-w-out-authorization