Hello. The forum already has answers to similar questions.
You can enable facebook login for your site, by going into admin panel and opening the Plugins tab. Search for Facebook on that page.
Here are two links that might help you:
http://www.question2answer.org/qa/10996/how-do-i-create-a-facebook-login
Additional reading - http://www.question2answer.org/qa/2953/facebook-connect-implementation
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