NEW YORK (CNNMoney) -- The news stormed across blogs and headlines this week: Facebook had become the first website to rack up a mind-boggling 1 trillion monthly page views.
The tip came from an advertising service called DoubleClick AdPlanner -- which, ironically, is operated by Facebook rival Google (GOOG, Fortune 500). The most recent AdPlanner report, which covers the month of June, showed that users viewed approximately 1 trillion pages on Facebook during the month.
But a different pageview measurement service, the widely used comScore, has a wildly different number. Its latest report, for July, found that Facebook had "just" 467 billion monthly page views.
Facebook still blows away the competition, with Google in a distant second place. But comScore's number is less than half AdPlanner's.
Why the discrepancy?
Measuring Internet traffic is an extremely imprecise art, and each tracker uses its own combination of metrics. The DoubleClick planner incorporates directly measured traffic from sites that use Google Analytics (Facebook presumably doesn't) and aggregated sample data from Google's other products, like the Google Toolbar that millions of Web surfers have installed. Google says it also factors in other third-party market research.
ComScore (SCOR), on the other hand, extrapolates total page views from two sources: a network of 2 million users worldwide who allow the company to anonymously record their Web traffic, and data collected from over 1 million domains.
Analysts say comScore's methodology is generally more trusted. The 12-year-old company makes money from selling its service to clients, so it's got a vested interest in paying rigorous attention to the accuracy of its methodology.
By contrast, AdPlanner is a free tool for advertisers who use DoubleClick. AdPlanner may help advertisers decide where to put their ad dollars, but it is not meant as a replacement for other measurement services.
Google admits as much, telling advertisers on its own explanatory page that "you can get an idea of" the pageview numbers from its data. AdPlanner's pageview counts are rounded to the nearest 1 million users; comScore's numbers are far more exact, rounded to the nearest 1,000 users.
"I'd think comScore's edge comes from the fact that it's in the metrics business, and its metrics are more widely used for ad currency and planning by large media buyers," said Andrew Frank, analyst at Gartner. "1 trillion does make for a good story, though ..."
Facebook declined to comment on whether it believes it has reached 1 trillion page views.
In June, Google became the first website to hit 1 billion unique visitors per month, according to comScore. That measurement counts the number of people who visit each site in a given month, excluding return visits. Facebook is currently third on that list, by comScore's count. It has 737 million unique visitors, behind Microsoft (MSFT, Fortune 500)'s 899 million. 
