Remember when web site traffic was talked about in terms of "hits"? You'd read about how many millions of hits Netscape got per month and other sites bragged about getting 30,000 hits a day. Eventually, we moved away from the term hit because everyone realized it was pretty meaningless. You see, a hit was often counted (depending on who was counting them) not just for a page load, but for every element (e.g., graphic) included on the page, as well. One visit of this page, for example, would be worth about 40 hits (if the browser had images turned on). But a site that was less graphical and had equal usage would register half the hits.

Pageviews replaced hits as the primary traffic metric not just because they're more meaningful, but because it also determined how many ads could be served. Ads were sold primarily on a CPM basis, so multiply your CPM by every 1,000 pageviews you got, and that's your dot-com revenue.

Reach (number of unique visitors) is also important, of course. comScore/Media Metrix uses uniques as its primary metric, because mainstream advertisers want to reach a lot of people, not just the same people over and over. You can also get pageviews, time spent, and several other data points from Media Metrix, but if you're the number one site on MM, it's because you have the most unique visitors for the month.

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