The new development was first caught by digital inspiration and later confirmed by Matt Cutts who tweeted, “Googlebot keeps getting smarter. Now has the ability to execute AJAX/JS to index some dynamic comments.”
This does not mean that the Googlebot has made its way into Facebook and can now read and index comments on your private Wall; however, if you use your Facebook to log in and comment on a favorite website, or on one of Facebook’s public pages, that comment can be traced back to you through a Google search. Besides Facebook Comments, services such as Intense Debate, Disqus and Livefyre can be indexed.
Digital inspiration illustrates the implications of this, using search queries such as “commenter name * commenter title”, which will hunt down all the comments made by a user who signed in using Facebook comments.
Privacy-wise, you probably needn’t worry as you’ve just read this handy little warning (warn your friends), and the plus side is comment trolls may have to do a different dance. It’s not certain how Google will approach older comment content though.