AdWeek's article at
adweek.com/aw/content_display/ ...

"....Over half of respondents said they search using multiple engines. Just 20 percent use Google exclusively. Yahoo is the exclusive engine for 8 percent, AOL by 3 percent and MSN a piddling 1 percent.
  
Forrester surveyed more than 4,800 U.S. online adults about their search habits. Their activity was self-reported, not observed.
...."