At Drew Houston's Startup School talk he mentioned something almost in passing which I think summarizes exactly why every investor wants or should want a piece of the filesharing pie: these systems have the opportunity to become the "filesystem of the internet". In this world of systems such as iOS and Chrome OS where the local filesystem becomes abstracted away, and while internet connections become increasingly ubiquitous, it isn't unreasonable to expect that "uploading" files to a website will really mean linking to a third party where those files are kept and "downloading" files will really mean pointing to the place where you want said third party to keep them. If that were to happen, and there existed a dominant player in the space who wasn't already a Google or a Facebook, they would quickly become one.