According to Compete data there are roughly 7.5 billion search queries performed every month by the US Online Population. However there are only about 5 billion search referrals every month

Yahoo! pretty much takes the cake on this one with about 75% of searches performed on Yahoo! in August resulted in a referral. By comparison, searches on Google result in a referral about 65% of the time and searches on MSN/Live result in a referral about 59% of the time.

it is important to consider actual referrals as well as top level searches