"Not only do Google users conduct more searches a month compared to those who use Yahoo and Microsoft, but they are also more loyal, according to a new report by ComScore with US-only data. The analysis examined the use of the various alternative search engines by users of the three search engines, revealing that those who searched on Google had the highest loyalty rate, with the majority of all their searches occurring on Google Sites. Users of Yahoo and Microsoft sites still conducted a higher percentage of their searches on Google sites."
source: http://arstechnica.com/microsoft/news/2009/08/google-users-more-loyal-than-bing-and-yahoo-users.ars

That's because generally the people who use Yahoo use it out of habit, and the people who use Bing use it by default in IE.

That's not true for everyone, but for a large percentage it is.