Why Facebook, ComScore disagree on users' ages

Sound the alarm bells: CPM Advisors has uncovered a drastic disparity between the demographics Facebook offers advertisers and the metrics ComScore independently reports. ComScore reports that 13.6 million U.S. people ages 35 or older use Facebook. Facebook, however, puts that number at only 1.26 million. What gives?

Facebook and ComScore have yet to elaborate on the big discrepancy. But there's one possible explanation rooted in ComScore's panel-based methodology. The other big discrepancy noted by CPM Advisors was in the 18-24 demographic; Facebook counted 50 percent more users than ComScore did in that age bracket. Here's a possible reason why: ComScore's software tracks the sites a single computer goes to. If a parent gives his kid an old laptop to take to college, that's instantly someone in the 18-24 demographic miscounted in the 35+ category.

Inside the advertising community, this is a well-known problem: In September and October, as students return to school, the numbers from ComScore, NetRatings, and other panel-based measurement systems become inaccurate. Over time, the panels get corrected, and ComScore's numbers return to their usual level of accuracy. For people in the know, CPM Advisors has uncovered exactly nothing.

But it does raise another issue. With the rise of sites like Facebook, where users have motives to report their age and gender accurately, it's not clear how much longer advertisers will tolerate these seasonal discrepancies. The promise of online advertising is that media buyers won't have to treat numbers as a matter of faith. Who can you trust? In that answer lies dollars.