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please show me How to add facebook functionality in drupal site?
Its a complicated topic, because the Facebook APIs can do quite a bit. I recommend you read up, starting at developers.facebook.com. You should also install the Drupal for Facebook modules. Read the readme and browse the issue queue on Drupal.org.
Then post here when you have specific questions, like "how do I post to a news feed?" or "how to authenticate via Facebook Connect?"