U.S. online ad revenues reached US$5.8 billion in the first quarter this year, up 18.2 percent over the same period in 2007, according to the Interactive Advertising Bureau (IAB).

However, it was the first time in three years that online ads experienced a sequential decline, lower than the highest $5.9 billion in the last quarter of 2007. The 18.2 percent gain, although healthy, shows that online ad growth has been slowing over the past year, paidContent reported.

Partner of assurance at PricewaterhouseCoopers David Silverman, who helped prepare the IAB report, explained the sequential decline may be caused by the traditional drop in ad spending after the holiday season, as well as an overall economic slowdown.

The IAB report accords with other reports released this month on online ad growth. The Nielsen Monitor-Plus' report showed that online ad impressions in the first quarter grew by 14.7 percent versus last year's 31.9 percent, and the National Newspaper Association said newspapers Web sites' ad revenues gained only 7.2 percent in the first quarter versus the 22 percent growth the year before. TNS Media Intelligence found that display advertising sales plunged sharply in the first quarter from the double-digit growth rates in 2007, but are still expected to reach a healthy 8.5 percent gain, PaidContent reported.