The conventional wisdom in marketing is that online advertising will continue its growth. Just today, the chairman of WPP, one of the world’s largest advertising agencies, said this about online advertising:

Companies are spending up to 12% to 13% on digital. We know we spend up to 20% of our time online, so in theory spending should be up at 20%. It’s just natural conservatism, resistance to change and inability to adapt to change. I believe when digital budgets get to 20%, which will be in four or five years, we will be spending 30% of our time online.

Apparently, Citigroup does not subscribe to conventional wisdom.

While large banking companies like Bank of America, American Express and Capital One Financial Corp. have been increasing their online ad spend, Citi has ratcheted it down noticeable. Last month, Citigroup fell out of the Top 10 among financial services firms based on image-based ad impressions online, according MarketingCharts.com. Compare that to August 2008, when Citi was the No. 3 online financial services advertiser with more than 1.7 million impressions in the month. That means Citi cut its online ad budget last month by at least 60%. On the other hand, Amex increased its online advertising last month by at least 48%, compared to July 2009.

Now, there are rumors that Citi is prepping a big ad campaign, and the bank may have scaled back its online advertising last month in preparation for that big push. We’ll find out soon enough.