All three engines have been using a variation of link analysis to determine what's relevant and what gets ranked into their databases. But the news of the week has been Microsoft's research into BrowseRank -- analysis that includes time spent at a site or page -- added into the source and number of links to the content.
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The more visits of the page made by the users and the longer time periods spent by the users on the page, the more likely the page is important.
Full story at http://searchenginewatch.com/showPage.html?page=3630408