I never use content on Yahoo and find parked domains in my logs that are part of Yahoo's search network.
I found that any site with search box can be a part of Yahoo's search.
I always wondered about low CTR on yahoo compared to other big two. Seeing my ads on Yahoo News, Yahoo that and Yahoo this, plus those sites that are nothing but fraudulent, I hardly use it today.
Iíll consider Yahoo only when Yahoo Search shows my ads on search.yahoo.com only, period. The rest should be clearly optional.