I would guess that Google handles about 80% of search engine traffic according to articles I've read.
I have no idea what the percentage could be, the only I know is that I keep using Google despite that everyday is delivering worst and worst results, but when those search results are extremely irrelevant or privilege the useless SEO optimized sites that often are on top, I move on to Yahoo and often find what I'm looking for. Just my actual hosting deal came from Yahoo, because Google is stuck in the old-same results, hosting services that I have already used in the past.
I wish i had numbers to give. but honestly, google is way better than Yahoo.. never heard of Yahoo's SEO either. bt there are a no. of new search engines coming up who draw their results from all these powerful big search engines and curate them.