Yahoo is now entering the context-driven ad field to compete with Adsense, and that's good for publishers and web authors. I have had great luck with adsense, and it provides serious money for this blog and my other web sites, which is why I have the affiliate link on the right bar encouraging you to join adsense -- though I won't mind the affiliate fee as well, of course.

But I'm trying Yahoo now, and soon MSN will enter the fray. However, it seems to me that no one network will be best for a diverse site. Each network will have different advertisers bidding up certain topic areas. In an efficient market, advertisers would quickly shift to the networks that give them the best performance (cheapest price, most qualified clicks) but in practice this won't happen very often.

So it would make sense for somebody to build a web site optimizing engine. This engine would automate the task of switching various pages on a site between one network and another, and measuring performance. Over time it would determine which network is performing the best for each page or each section of the site and switch the pages to use the best network. It might run further tests to see how things change.

Such optimizations could take place even during the day. (Yahoo doesn't have much intraday reporting yet.) For example, Google does better in the morning than it does in the evening. I guess that this is because advertisers have set a daily budget, and more of them hit their budget as the day goes on. My CPMs usually start high and then sink in the later hours. It might make sense to switch from Google to Yahoo as the CPM drops. However, Yahoo's advertisers will have their own budget limits so this may not help.

Another interesting optimization might be to present different ads depending on whether the user came in from the associated search engine. Theory: If the user searched for "copyright" on Google to come to my copyright myths page the chances are they already saw a lot of copyright related adwords ads. Might make more sense to show a different set of ads from another network. Likewise if they came in from Yahoo, might be best to show the Google ads. If they come in from elsewhere, use the best performing network. This would be generated live, based on the Referer field. Hard to say if the search engines would like it or not