I first noticed the popularity feedback loop back in the BBS days. A common feature of BBSes was a "Most Popular Files" list, and people just kept downloading the files on the list, which to my mind defeated the purpose of advertising these files. I set up a "Least Popular Files" list that listed all the files that had never been downloaded. It didn't really work. 
Sun Mar 05 2006 23:22:
Shuffling a Stacked Deck has a good approach to a popularity feedback loop. The solution is to randomly insert certain results into the list of the most popular. More elegant than the Ultra Gleeper's solution of multiple tiers of results.
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