Yahoo is the second leading pay per click platform. SEO consultant, I've spent years working with Yahoo's marketing program and was a certified Yahoo Ambassador when this program was offered. As they are being integrated with Bing's program I can assist with the transition.
Part of my search engine marketing services is to help clients decide on whether their ad spending is best utilized. Google has the largest reach, but their average cost per click has risen dramatically the last few years. Yahoo, Bing, and other search engines may be a better value.