This algo update will cause a major change in the rankings of many websites. Google also cleared that this update does not depend on the feedback they have received from Personal Blocklist Chrome extension, but they have compared that Blocklist Data with the sites identified by this algorithm.
In February, this algorithm update was launched for US only and now on April 12, 2011 this algorithmic improvement is launched globally to all English language sites.
Checkout what Google said about this major update:
“Today we’ve rolled out this improvement globally to all English-language Google users, and we’ve also incorporated new user feedback signals to help people find better search results. In some high-confidence situations, we are beginning to incorporate data about the sites that users block into our algorithms. In addition, this change also goes deeper into the “long tail” of low-quality websites to return higher-quality results where the algorithm might not have been able to make an assessment before. The impact of these new signals is smaller in scope than the original change: about 2% of U.S. queries are affected by a reasonable amount, compared with almost 12% of U.S. queries for the original change.”
Google has tested this algorithm and found it very accurate in finding quality sites. If your site has been affected by this update, then analyze various aspects of your site. You can review Google’s quality guidelines to get helpful information about improving your site’s quality.
Google has not provided the names of any websites which went down after the effect of this algorithmic change; Google’s main focus is content farms. Through this algorithm Google will focus on websites which are content farms that uses copied or duplicate content. So if your site is using any content farm, be focused otherwise your site will be go down.
According to Amit Singhal they internally call it "Panda" (name of an engineer among the key guys who made this update possible.) Danny Sullivan of Search Engine Land is calling it “Farmer” as this algo has targeted content farms.
see our Google seo updates 2013