Online advertising for the first nine months of 2007 was up 15.9 percent, according to Nielsen Monitor-Plus’s latest figures. While still the leading gainer compared to other media, online’s growth rate is much slower than 2006 when internet ad spend grew 49.2 percent through Q3. For all of 2006, web ad expenditures were up 35 percent, Nielsen reported in March.

As every measurement of internet advertising has shown this year, growth has been steadily declining. In its Q1 survey, Nielsen said online grew 31.9 percent. Then, for the first half of the 2007, internet ad spend number were up 23.2 percent.

Earlier this month, TNS Media Intelligence reported that spending on display ads for the first nine months of 2007 came in at 17.2 percent the same period last year. TNS, which doesn’t measure paid search, noted only a small difference between that number and the 17.9 percent growth measured comparing Jan.-Sept. 2006 and the same time frame in 2005.

The slowing growth has been pinned on the “law of large numbers” that says high growth can’t be sustained over long periods. Others say the lack of comprehensive metrics has held ad spending back, a theme that’s been sounded by the Interactive Advertising Bureau and its members over the past year. Overall, the ad industry has teetered on the brink of an ad recession lately, as the credit crisis and wider concerns about the health of the U.S. economy have continued to spread. And while most reports have shown advertising as a whole flat to up slightly, the current Nielsen report found that ad spending for the first nine months of 2007 slid into negative territory at 0.1 percent. Release

• Display Ad Revs Rise 17.2 Percent In 2007’s First Nine Months; Growth Rate Slows: Report