The quoted number sources just show how bollocks stats can be.
The "unique visitor" count is useless. It does not show usage patterns at all. If I hit Google 5000 times and Yahoo once, then thy both score 1 "unique visitor" point.
Ranking is also a bollocks measure, only actual numbers count. The real number is in the Nielsen report: Google out searches Yahoo 4:1. Yahoo hardly "came second". You don't get an Olympic silver medal for taking 4 times as long as the winner to run the 100m dash.
What is more important is the trend. Google is getting bigger and Yahoo is getting smaller.