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Web metrics firm ComScore says it going to begin tracking ad networks' "potential reach" and "actual reach" for online-ad buyers and sellers. A translation: Ad networks, in theory, can place ads on all of a Web publisher's pages, and those are the numbers they trot out when luring ad dollars. Operations like Glam Media compound the confusion by portraying some of the sites they represent as if they were websites they owned. In practice, publishers of a respectable size use networks only to fill a small percentage of their least valuable inventory. The net effect: industrywide, advertising inventories look much larger than they actually are, leading ad buyers to drive harder bargains. If ComScore can expose this part of the ad-network scam, publishers may benefit from higher rates. Ad buyers? They won't complain: As soon as they've spent their clients' online budgets, it's time for a two-martini lunch..