A massive boost for why you should be using an add-on commenting system like Facebook comments, Disqus or something similar for your blog – Google now crawls these comments.
So if you are talking to the right people, generating decent traffic, and getting the keywords in your comments as well, imagine the search engine potential you are giving yourself.
Just note that this applies to third party commenting applications, off of Facebook. So for example Disqus (like I mentioned) and the Facebook comments plug in. Comments on content which are made on Facebook itself, don’t count for these purposes.
Have a look at the following screenshots by Robert Scoble, showing this in action. This info was originally posted on www.readwriteweb.com :
This was the article:
This was the comment made: