Over the last 10 months, Google has rolled out an updated algorithm that has been dubbed Panda or Farmer by the SEO community. In simple terms, the update was intended to help weed out spam or weak web sites and help Google searches to produce better, more qualified web site results. Matt Cutt's Google head of spam, explained it like this:
"This update is designed to reduce rankings for low quality sites—sites which are low-value add for users, copy content from other websites or sites that are just not very useful. At the same time, it will provide better rankings for high quality sites—sites with original content and information such as research, in-depth reports, thoughtful analysis and so on."
However, as with any Google update, there is always collateral damage. In this case, the organic (non-paid) rankings for many high-quality web sites have been affected. That said, there are various methods that can be applied to help thwart the negative rankings and reduce the effects of the Panda update. If you've see a drastic reduction in site visitors, especially in the US, over the past 6 months, we highly recommend that you have a an expert SEO person check your site to identify if you have been affected and make recommendations as to how to address and correct these issues.