According to John Battelle, Google is testing a Cost Per Action service (CPA) - the original source is SeekingAlpha.  Here’s some excerpts from SearchAlpha

"How do I get paid?

You get paid whenever a site visitor clicks on the ad on your site AND performs a specified action, such as generating a lead or purchasing a product.

Do these compete with regular content ads?

These ads will not compete with contextually targeted ads. Instead, they will show across a separate network, the Content Referral network. To place one of these ads on your site, you can set up a new ad unit that supports any of our current ad unit sizes.

How much could publishers expect to earn with this CPA test?

How much a publisher will earn will depend on a number of factors about the publisher and advertiser, including whether the ads match the topic of the site, and level of interest of their site visitors. We have tried to match the appropriate publishers with advertisers for this test.

Will CPA offerings compete with my current AdSense revenue?

We expect that the CPA test will offer ad units that will expand publishers AFC revenue because the ad units are separate and appeal to different types of users. These CPA ads are also additional inventory to your existing AFC ad units.

How can I promote the CPA ad unit?

Since this is a test and these CPA ads are not regular ad units, we are giving you more flexibility in saying things like “I recommend this product” or “Try JetBlue today” next to the CPA ad unit. However, you should still not incite someone to click on the ad, so saying “Click Here” is not ok.

Where do these CPA ads comes from?

The CPA ads come from a limited group of high quality advertisers that are interested in displaying ads on a CPA basis. They pay you whenever a site visitor performs a specified action, such as generating a lead or purchasing a product.

Will I be able to see reports within my account?

When the test begins, you will receive weekly email reports of conversions you have accrued and your total revenue within the CPA test.

That’s going to work much more effectively on Google’s Network than SNAP - it’s the Google’s size and mainstream use of it’s search engine that is going to drive this.