Paid Content has a great article about online advertising and how the concentration of online advertising in the hands of so few websites is becoming a problem. 

They note this remarkable stat from Zenith regarding distribution of online ad revenue:

So the big problem is not that ad spending is drying up, it’s that the bulk is concentrated in a few sites. Citing the IAB, Reuters points out that the top 50 websites in the U.S. took in more than 90 percent of the revenue from online ads in H107, while the top 10 sites sucked up 70 percent of internet revs for the same period. 

They also quote Zenith as suggesting that even as late as 2009 online advertising will remain a fraction –  under 10% – of the total global ad spend of some 495 billion.     I’m skeptical of that estimate – very skeptical – because online ROIs remain vastly superior to offline, though this advantage is not as obvious as it should be because so much of the spend is done in foolish “old media” ways with large, expensive, poorly targeted campaigns.  As PPC campaign sophistication improves, people continue to move online, video continues to move online, and advertisers increasingly continue to insist on positive ROI we should see online buys approach offline – I’d wildly guess there will be online / offline ad parity by 2015, though interactive TV and video clip advertising may blur the distinction between a TV ad and an online ad.