The Comments plugin 
 lets people comment on content on your site using their Facebook account. If people wish to they can share this activity to their friends and friends of friends in News Feed as well.this post will guide you how you can get facebook comments (plug in) box in blogger.

1-Go to   
https://developers.facebook.com/docs/plugins

 click on comments                     Direct link

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2. Choose URL or Page

Pick the URL of a website you want to use with the comments plugin. for example,
http://zoyaxmalik.blogspot.com

3. Code Configurator

Paste the URL to the Code Configurator and adjust settings like the width of your comments plugin or leave it blank and how many posts you want to show by default (5). Click the Get Code button to generate your comments plugin code.

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3.  Open your blog and click on Template, new  window will open click o EDIT HTML

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4. press ctrl+F button to get the search box in HTML page, type the following command in search box.

</body>

you can copy this form here and paste it into your HTML page's search box.