"So how many people read your blog?" It's a question I get asked, but it's actually very hard to answer. There's no standard counting method and different traffic-monitoring services give different answers. Of course in some sense it's always been the case that nobody really knows how many people read The Washington Post or watch The Sopranos, but the internet has created an odd combination that provides the illusion of server-derived precision (by one standard, at least, precisely 669 people clicked on the individual page of my "Ghost of Grover Cleveland" post on Friday, generating 844 distinct page views) with the reality that there's simply no widely accepted industry-standard counting method.
The upshot, as Louise Story writes is to substantially retard the development of online advertising and throw the whole future of media into doubt.