The Interactive Advertising Bureau reports that online classifieds revenues have fallen by about 31 percent in the first half of 2009 in the U.S., the result of which dragged down total online revenues in the first six months.

Total Internet advertising revenues in the U.S. were $10.9 billion for the half year, the report said, with Q2 accounting for about $5.4 billion. According to IAB’s report — prepared by PricewaterhouseCoopers — U.S. classifieds were pegged at $1.116 billion in the half-year that ended June 30, a $495 million fall from fiscal half-year 2008, or $1.611 billion. In FH 2008, online classifieds comprised 14 percent of online ad revenues, PwC said; for FH 2009, that fell to 10 percent.

While we’re all for conservative measurement, we think the half-year, billion-dollar figure for classifieds is unrealistically low, even in a sucky economy. In the report’s methodology statement, it defines classifieds as inclusive of fees paid to job boards, employment listings, real estate listings, automotive listings, auction-based listings and yellow pages. 

The underreporting of classifieds in the IAB / PwC research has always puzzled us. Monster alone showed revenues of $1.1 billion in 2008. EBay earned more than $8 billion last year. While Monster and EBay are global, even if only half their revenues were domestic, those two companies alone would account for the approximately twice the $2 billion PwC calculates as the online classifieds total. Move.com — entirely domestic — earned nearly a quarter billion alone last year.

Nevertheless, the downward trend is real, although we’re not as certain that it’s as steep as IAB / PwC report. We’ll know more when public companies file their Q3 reports.

The IAB report said that for the first half of 2009, total online ad revenues declined 5.3 percent from the same period of 2008.Search and display-related advertising continued to comprise the largest slices of the ad pie for the half year, at nearly $5.1 billion and $3.8 billion respectively. Search revenues were slightly up; display revenue — which includes display ads, rich media, digital video and sponsorship ads — was down 1.1 percent.

“We are in one of the most difficult economic slumps in decades,” said Randall Rothenberg, CEO and president of the IAB.  ”In recent years the digital revolution has driven a transformation of how consumers experience advertising and media.  As the economy improves, we’re confident that brands will devote an even greater share of their budgets to reaching consumers as they make interactive media a larger part of their lives.”

According to the report’s boilerplate: 

“The IAB sponsors the IAB Internet Advertising Revenue Report, which is conducted independently by the New  Media Group of PricewaterhouseCoopers. The data is compiled directly from information supplied by companies selling advertising on the Internet. The survey includes data concerning online advertising revenues from Web sites, commercial online services, free e-mail providers, and all other companies selling online advertising. 

“The full report is issued twice yearly for full and half-year data, and top-line quarterly estimates are  issued for the first and third quarters. PwC does not audit the information and provides no opinion or other  form of assurance with respect to the information.”

A copy of the full report is available here.