How Copyright Makes Books and Music Disappear (and How Secondary Liability Rules Help Resurrect Old Songs) (http://www.boingboing.net/2013/07/05/how-long-copyright-terms-make.html)
with links to this paper
“An analysis of the data demonstrates that the DMCA safe harbor system as applied to YouTube helps maintain some level of access to old songs by allowing those possessing copies (primarily infringers) to communicate relatively costlessly with copyright owners to satisfy the market of potential listeners.”