They may be the biggest and most powerful public companies in the U.S., but the Fortune 500 have a thing or two to learn about search engine optimization. A new study released by SEO technology company Conductor that these companies continue to be “largely invisible in natural search.”

The study titled “Natural Search Trends of Fortune 500 Q4-2008” analyzed approximately 90,000 primary paid keyword on which the companies were advertising and examined how well those companies ranked naturally on those same key words. The findings were stunning.

While the companies are spending $51 million per day on paid advertising, they only rank in the top 100 natural search results for 21% of the keywords. In addition, the study concluded that 47% of the top 500 public companies in the U.S. have “very low or non-existent visibility for their most advertised keywords.”

Talk about the need for a thorough review of their current search engine strategy. This study confirms that all companies, big and small could benefit tremendously from a strong push on increasing their organic search presence.