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

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 a开发者_JS百科uthenticate my Facebook app?

All I need is some facebook-related-identifier, it does not even have to be the User Id, but any kind of identifier that can relate the user to a facebook profile.

Ideas?

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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.