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Installing the plugin is simple: if your website has the Comments Box plugin installed, the mobile variant will automatically appear on the mobile version of your site.
The comments can be moderated, and Facebook is using a special "social relevance" algorithm to detect the most relevant comments for each user.
The Comments Box, argues Facebook, is useful since it's adding to the quality of the conversation as well as social relevancy and social distribution. Facebook's best practices for implementing the Comments Box on your website can be found here (PDF link).