Recently, people started noticing Google being able to index Facebook comments without problem. The hard part is that those comments are rendered in AJAX and only visible via POST requests. GoogleBot, Google's spider, was not able to index that type of content on the web before.
It is widely used for blog comments, such as Facebook comments and Disqus comments.
Matt Cutts confirmed this on Twitter a few hours before the Google blog post:
For more details on the technical stuff around this, see the Google blog post.