Wikipedia has many low popularity articles that, collectively, create a higher quantity of demand than a limited number of mainstream articles found in a conventional encyclopedia. The same could be said for Amazon's book inventory or Netflix's movie inventory. The total volume of low popularity items exceeds the volume of high popularity items.
The following book describes the subject in detail:
The Long Tail: Why the Future of Business Is Selling Less of More
Or check out this blog for intro: Long Tail 101
