Get Facebook User Id of my website visitors w/o auth

Question

Is there any way to get the Facebook User Id of visitors in my website by using the FB API, but without need to ask them to authenticate my Facebook app?

All I need

Answer 1

If the user does not explicitly authorize you to get some private data from his Facebook account (included his Facebook ID), you can not get them.

I cannot think of a way to get the Facebook ID without the authorization of the user.

Hope that helps.