Yes, I know Yahoo and Microsoft got approval to move forward with their search deal (I'll link to my post on that, when I write it, in about 10 minutes from this post), but Yahoo Search Marketing has been a failure for many in the past few months. And since Yahoo Search Ads will likely continue to be powered by Yahoo until December of this year, this is still important.
A WebmasterWorld thread has posts from several disgruntled Yahoo Search advertisers. They are all pretty much complaining about the same thing - search volume. Yes, the number of people seeing and clicking on their search ads, compared to the previous years, has dropped significantly. Let me pull out some quotes:
This year we notice Yahoo PPC struggling to generate decent volume for us. Its getting worse and worse, and all the new measures they are taking to ensure good quality traffic are infact killing our traffic.
I think part of your PPC woes are exemplified in that YPN payouts completely died for me, removed it, which I'm sure many others have done, thus eliminating millions of page impressions previously available.
For the last two months we underspent our budget, so I have noticed this too.
Yahoo has been fully aware of the Microsoft deal - so why invest? Yes, Yahoo has added features recently, but those features should have been added over a year ago. In any event, by years end - this likely won't matter anyway. More on that shortly.
Forum discussion at WebmasterWorld.