There are thousands of apps built on top of Twitter, but are any of them a true killer app that could give rise to an entirely new sub-industry? Venture capitalist, and Twitter investor, Fred Wilson doesn't think so. While Twitter desktop clients, mobile apps, photo uploading services, and URL shorteners have all made Twitter what it is today, he points out : Much of the early work on the Twitter Platform has been filling holes in the Twitter product. Wilson is still waiting for the equivalent of the desktop publishing startups which defined the early Mac computer or Lotus Notes for the PC. The time for filling holes is over, he suggests.