A host of companies are doing their level best to keep tabs on how many people are visiting a given Web site, and what those users do while they are there. Some use tags installed on a user's Web browser, while others extrapolate surfing habits from a sample audience. A handful use a combination of several methods.

Here's a glance at a range of approaches, with a look at what Alexa, Compete and Comscore have to say about one popular online video site, Metacafe.

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