I suspect that an economic downturn if it does come will be good news for online advertising. Why? Only because there is nothing quite like a downturn to accelerate efficiency. Last time round, the recession was great news for Indian IT companies -- simply because it became efficient for American companies to outsource certain things that they do. Though I dont have figures to back it up, there are quite a few articles out there that point out that the following 2 ratios are not the same (which by implication points to an inefficiency in the allocation of advertising dollars):

Time spent by people online:Time spent by people on old school media Dollars spent on online advertising:Dollars spent on advertising in old school media

The 1st ratio is of course larger than the second.