There have always been affiliate programs like CJ. There has always been PPC search like Goto (Overture).
But when advertisers want to reach specific audiences they have often turned to advertising on content sites.
www.concrete-home.com is an example of such a content site.
www.surebaby.com too.
But now Google steps in with Adsense, with a huge reach and low prices. I've seen with my own eyes advertisers stepping away from site sponsorships and turning to Google Adsense.
The monthly revenue of content sites may go up, or it may go down. I have a feeling it will go down for many. More and more sites are being put up specifically to cash in on Google Adsense. These people wouldn't even be online if it weren't for the potential income from Adsense.
As more and more content sites go up, they will lower the value of a click, and consequently the earnings of content sites will drop.
I see this as a threat. Google may very possibly be causing a recession amongst content site owners.
Adsense may very well be good for advertisers, but not for publishers.