As the rest of the media economy slows down, online ad revenue continues to grow, albeit at a slower pace.

Third-quarter revenue climbed to $5.9 billion, according to numbers released this morning by the Interactive Advertising Bureau and PricewaterhouseCoopers, the second-best quarter ever recorded.

That was up 11 percent over third quarter 2007 and brought the year-to-date total to a record $17.3 billion, 14 percent better than last year’s total at this time.

Though third quarter has proven quite rough for a variety of media, including magazines, radio and newspapers, online ad revenue was still up 2 percent over second quarter’s total.

PricewaterhouseCoopers predicted that, while the weakening economy will continue to challenge ad-supported media, the internet is in a better position to weather the slowdown “given its ability to combine performance-based advertising along with broad-based branding.”

The nine-month growth pace for the web is well below those of the last two years. Last year finished up 26 percent and 2006 was up 35 percent, but it's still tracking slightly ahead of forecasters' projections. Magna’s Robert Coen had pegged 2008 online growth at 12 percent.

And compared to other media, the internet is holding up extremely well. During the first half of the year, overall ad spending dipped 1.6 percent compared with last year, according to TNS Media Intelligence.

Third quarter could get a bit of a bump because of Olympic and political ad spending, but forecasters have said they will not hit nearly the level that many had anticipated a year ago.