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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.
Now, Google announced they have "started experiments to rewrite POST requests to GET." They still recommend using GET requests but for AJAX and some JavaScript becoming really popular, Google knows the importance of crawling this content.
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.
Forum discussion at WebmasterWorld, Sphinn and Google +.
