While popular conceptions of the LongTail of NicheMarket?ing often talk about "infinite shelf space", the reality is of course that there really is a finite shelf space, even in the digital environment. The shelf space is merely very, very large compared to its RealWorld namespace (far beyond an AnthropologicalLevel?). In the PowerLaw distribution of popularity vs. inventory, after a certain point the resources of the retailer are fully accounted for, and so the LongTail must drop off to zero. This drop off is called the DroopingTail because it is usually very sharp.
The DroopingTail can occur in two cases. In the first case, the available shelf space far exceeds the available stock. For instance, while web servers can cheaply serve millions or even billions of distinct web pages, creating that many pages is very difficult. The bottom end of the pages will very drastically become unpopular on the curve. This suggests that while adding more content will extend the size of the LongTail to capture more of the bottom end of value, after a point the cost of adding new content exceeds the total expected value of what would have remained of the LongTail .
In the second case, the available stock may exceed the capacity to serve it to customers. For instance, while there are many movie theatres, there are far more movies. The PowerLaw distribution of popular movies hits a point where the LongTail turns into a DroopingTail simply because there aren't enough screens to show the entire inventory.
The consequence is often that sites solicit TheAudience to become TheAuthor (through OnlineCommunities) in order to cheaply supply additional content.