The most important is your ranking in Google while in Yahoo is also one of the search engine,I think you must concentrate in Google ranking..
Focus on yahoo and bing - they are not dying, just restructuring - focus on building backlinks and having keyword rich titles and URLs
You should pretty much focus on Google, as it is rocking the Search Engine World and will keep to rock unless Yahoo and Bing tie up together. Yahoo is dieing day by day
Yup its the case with me to initially it was contributing 15% of my overall traffic. But now its dropping
Ranking in Yahoo is not so important. What's more important is, ranking at the Top Search Engine (Google). Most of the people ignores other SEs and focus more in Google Rankings.
If you're building a long term business focus your effort on Google. If you're looking to make fast cash via adsense or affiliate marketing, move your efforts to yahoo & bing. You can see from all the comments above, no one bothers with yahoo & bing so this makes it very easy to dominate areas quickly. Build links... lots of links... then more links... then some more.. What works for one engine doesn't alway work for another and these things change quickly. Get an idea of what works, then rinse and repeat!