In the Algorithm research session at Search Engine Strategies today several panelists mentioned that search engines might use the length of time one spends on a result page to downgrade its relevance in certain circumstances.  If everyone immediately clicks back to the search engine for example quickly after visiting a page, the engines may take this as an indication of user dissatisfaction with the result.

In a similar vein, it seems like one could track the time between when one clicks a SERP result, and when a corresponding ad is clicked on the destination page as a possible indicator of spam activity.  There are so many pages now that are nothing but lifted content designed to get in the SERPs to earn revenue through adwords or similar ad networks.  If very quickly after the average visitor is sent to page, another ad is clicked, this might identify these ad-centric pages and allow for them to be moderated down or out of the results as appropriate. 

This of course is somewhat limited as the engine must match the ads being served to track the time difference.  Also, there is a conflict of interest in these situations, as the engines may themselves be benefiting from the revenue these sites produce.  In the long run though, getting rid of these sites will improve the user experience, and thus improve the value of the ad channel to potential advertisers.   So even if it’s value is somewhat limited, it might be worth trying to put something like this in place to at least guarantee the integrity of the ad network within the serving engine itself.