Media metrics firm Nielsen NetRatings has decided to concentrate on how long pages are viewed, rather than the number of page impressions, as a method of measuring websites' importance. 

The move is likely to hurt the rankings of search engines such as Google, since they receive a large number of page impressions but the actual time spent on individual pages tends to be rather short. 

Google topped the charts with over 110 million unique users in May but came nearly bottom in terms of time spent on the site, with 7.2 billion minutes compared to AOL's 25 billion. 

"The total number of minutes is the best engagement metric in this initial stage of web 2.0 development," said Scott Ross, director of product marketing at Nielsen's NetView service.

"This is because it ensures fair measurement of websites using rich internet applications and streaming media, and web environments that have never been well-served by the page view, such as online gaming and internet applications."

The top rated sites in May under the new system were AOL Media Network, Yahoo, and MSN/Windows Live. The bulk of minutes on the sites were spent on email and instant messaging. 

Nielsen's new system is expected to heavily favour web 2.0 sites like YouTube, where visitors spend a significant time watching streaming video. Online gaming sites are also expected to do well.