eBay and Yahoo have both recently changed how they pay the Publishers in their ad networks. The eBay Partner Network was a CPA-based affiliate network, up to this point. Their communications both talk about payout being based on the “quality” of the publishers traffic. Quality as determined by their algorithms. TechCrunch writes up the Yahoo Publisher Network changes here.
Changes like these put Yahoo and eBay at risk of losing Publisher site space to Google (AdSense) and other ad networks.
While most of the focus has been about the battle in search,Publisher Networks also are key competitive arenas for Yahoo and Google, as well as eBay and Amazon (with their Associates program). The advantage Google has in search carries over to their Publisher networks, due to the network effect.