Facebook Comments box tool.
The Facebook comments box is a social plugin webmasters can easily integrate into their website. Use the FB comments box tool to check the number of comments for a specific URL.
Facebook uses social signals to display high quality and most relevant comments for each user. When a user selects the option post to Facebook, friends can see the comment / story inside their news feed.
Facebook SEO advice:
By default the FB comment box is displayed inside an IFrame. To get the FB comments and make them accessible for search engine spiders, use the FB graph API and display to comments as content inside the body of your page. Use caching to reduce load time to improve page speed.
Access the Facebook comments box API: https://graph.facebook.com/comments/?ids={YOUR_URL}.
More information by Facebook on how to implement the comment box: http://developers.facebook.com/docs/reference/plugins/comments/
Just updated to Facebook API (2.7).