Google released Panda 4.0 (May 21, 2014). This is an SEO change which affects many sites.

Summary: Panda 4.0 moves up the ranking of pages with good quality and it lowers the ranking of pages with bad quality. If your site has good useful quality information, it may show up better in Google. But if it has blah-blah-blah info that you bought for $10 or $20, your page will drop (and your traffic will collapse).

How bad? eBay has a 33% drop in SEO visibility and fell 50%.

A good analysis with lots of data at SearchMetrics

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