• by Tameka Kee , October 22, 2007

Online ad impressions dropped by an average of 16% from August to September, according to data from Nielsen//NetRatings--with downward spending trends showing up in all but two of 13 verticals.

The category-specific view of the Web's biggest spenders in terms of impressions and ad types for last month point to what some analysts say may be the beginning of a macro shift fueled by the credit crunch.

Advertisers of B2B services tightened their belts the most when it came to online ads--as they paid for about 8.3 million impressions in September, down nearly 40% from August. Software marketers' impressions were down by almost 35% to 4.4 million, while public services advertisers paid for 9.6 million spots (a 25% dip).

Even the advertisers at the top of the pack bought fewer clicks, video plays and banner ads than in the previous month, according to Nielsen//NetRatings.

Financial services advertisers took the number one spot with just over 87 million impressions--a drop of almost 12% from August. Meanwhile, Web media took the second spot with just over 74 million (a 4% slip), and retail goods and services rounded out the top three with a little over 51 million impressions (and a less noticeable 0.04% drop).

Hardware/electronics and consumer goods were the only verticals that clocked in more online ad impressions in September--with increases of almost 14% and 9%, respectively.

As for preferred ad type in all of the verticals, the standard image and text combination took the top spot--with more than 166 million impressions served in September. Flash/generic banner ads came in second, with more than 79 million impressions, while sponsored links took third with over 65 million.