Pretty simple! First you need to create a Facebook App for your WordPress. You need to go to https://developers.facebook.com/apps and
Create an app with an App Name, for me it was blog.tahwidkhan.com
You at least need to provide these information for your blog app:
Make sure the App’s Sanbox mode is disabled, after you go live with your blog. You are almost done, now go to http://wordpress.org/plugins/facebook/ , download/install that plugin from your WordPress admin. You will find a Facebook item on your left navigation bar. Click that,
Provide your App ID and App Secret key that you got from your facebook app. Save it. Go to the Comment box from the WordPress Facebook plugin navigation.
Check where you want to show, post, page, attachment, check the other parameters. Save it!
You are done!!! 🙂 You can also do other stuff with Facebook plugin, e.g. Like Button, Send Button etc.