What does Facebook know that the U.S. Census Bureau doesn’t? 

I ask because the social giant is claiming that its Ads Manager can potentially reach 41 million 18- to 24-year-olds within the country, even though the Census Bureau only counted 31 million 18- to 24-year-olds last year.

What’s more, Facebook says its ad tool could possibly reach 60 million 25- to 34-year-olds and 61 million 35- to 49-year-olds, even though the Bureau estimates that there were only 45 million and 61 million of each group last year.

Pivotal Research analyst Brian Wieser spotted the problem after reading about similar discrepancies in Australian trade publication AdNews last week. Facebook has apparently been claiming a reach of 1.7 million more 16- to 39-year-olds than exist in that country.

What gives?

Well, we know Facebook has a history of overstating some metrics and miscalculating others. Last year, the Association of National Advertisers saidthe fact that Facebook had overestimated video viewing for two years was quite “troubling.”

 
Facebook, for its part, blamed the issue on a bug in Page Insights.

In this most recent case, the tech titan seems to be sticking by its figures. “Reach estimations are based on a number of factors, including Facebook user behaviors, user demographics, location data from devices, and other factors,” the company said in  astatement. “They are designed to estimate how many people in a given area are eligible to see an ad a business might run … They are not designed to match population or census estimates.”

That’s news to the agency executives with whom Wieser engaged in discussions.

“The gap between Facebook and Census figures is not widely known,” Wieser asserts in a note to clients this week.

What happens now?

Wieser, at least, doesn’t expect the latest revelations to convince many advertisers to stop throwing money at Facebook.

Yet, as the company continues to wade into original content,  Facebook's measurement issues "will help traditional TV sellers justify existing budget shares and … restrain Facebook’s growth in video ad sales on the margins.”

Oh, and Wieser predicts that Facebook’s funny math bodes well for third-party measurement firms like comScore and Nielsen, on which big advertisers will increasingly rely