According to a posting at eMarketer.com…
A WebTrends survey of 132 search marketers at the Emetrics Summit found that 81% ran paid search campaigns on Google. Another 64% ran campaigns on Yahoo!, 49% on MSN, 18% on Ask.com and 12% ran campaigns on other search engines…
Of course the best place to run your campaigns is the place that produces the best results. Just because “everyone” uses Google doesn’t mean you’ll get the best results there. Greater exposure to a smaller audience may deliver better results than so-so exposure to a larger audience. Do a little experimenting to find the right paid search mix for your offerings.