Google is an ad company, and a very good one. Yahoo is an ad company as well, but they are smart about social media.
However, the way people and consumers search is going to continue to shift away from Google towards a more social model (that is what Yahoo is positioning for with its recent acquisitions).
When this happens, CPC will explode and the split between CPC and CPA will finally be bridged.
Yahoo knows this, and so should you.