Isn't ad spending online already heavily weighted to "performance based"? Search and Google ad words are all pay per click which is performanced based and represents the lions share of online advertising. Google just picked up DoubleClick becase they were missing the 1/3 of ad spend in branding campaigns that is traditionally found in display ads which are usually sold on an impressions basis. I think the real question is will display (impression) advertising catch up to search (clicks) this year or not?