This snippet from the FAQ of Google’s PayPerAction page points to more than just an explanation of how it’s PPA system will work. This points to an explanation of why it will work…

Can I identify more than one type of action?
Within any campaign, you can track several different types of actions and pay different amounts for each one. For example, you could choose to pay $5 for a sign-up and $10 for a purchase.

Inventory, inventory, inventory.

Not just inventory, but diversified inventory that responds to different types of actions and can (and must) be measured with various yardsticks.

This type of cost per action marketing, in which several actions can be monetized and tracked from one location, flies in the face of the type of CPA that we normally associate with affiliate networks such as Commission Junction, Linkshare, Performics or ShareASale. This type of CPA marketing definitely differs from the CPA networks which generally use pixel tracking and restrict the types of actions that can be tracked at once.

This is active tracking and monetization of attention based in practical economic terms.  For everyone looking for ways to monetize web2.0 and social media… here you go.

The rapid expansion of diversified inventory on the web, and the ability to monetize that diversified inventory, will be the catalysts behind Google’s success with its PPA program.