Google Analytics uses a tracking pixel that a javascript snippet injects into a site so it can be downloaded by the browser. Because the gif is dynamically served by a server and there are a bunch of parameters in the request they know who you are, what the site was and then you have tracking. Knowing the actual number of people that read the post however isnt known as it is important so as to know how many people are actually ready the content and not just visiting. What do you think?