Any Facebook comment posted on any third-party website has been indexed by Google.
This simply means that readers' comments can affect Google PageRank of a website if Facebook Comment Box is used to empower the commenting system of that particular site's individual content. These days this trend of commenting through Facebook Comment Box is very common in news websites as well as blogs.
Search engines normally do not detects comments posted by users through various services like the Facebook Comments Box, Disqus, or Intense Debate.
However, ever since Google started indexing comments, the comments posted by users can be viewed in the results of a relevant Google search. This was first noticed by tech website Digital Inspiration.
Head of Google's Webscam team, Matt Cutts, declared on Twitter that, "Googlebot keeps getting smarter. Now has the ability to execute AJAX/JS to index some dynamic comments."
Facebook and Google may not be having strong professional bonding, but this news of indexing can be seen as good news for the social networking giant. Those who, in view of SEO criteria, were avoiding commenting through Facebook Commenting Box can now use this medium to voice their opinion.