Earlier quoted context omitted.
Yahoo didn't even use Yahoo! Mail which is much much much better than Facebook Messages. It was mainly because a lot of people were comfortable with Exchange and the Calendar integration. They tried Zimbra without success. At least they use Yahoo! Messenger internally.
During my time there a lot of the Engineers used Google Search, and a local Jabber install for IM because Y! Messenger was that ad-filled and painful to use. (Maybe my experience was limited to a pocket within Yahoo!)
I also used Google Search though I would try Yahoo! search every couple months. I don't believe the search engine is that much worse but I'd become accustomed to searching Google and understanding the results.