Which search engines are most popular?

This is a trickier question than it seems, actually, because there are so many ways to measure popularity. For example, do we just use Alexa numbers, knowing that it's a fundamentally flawed measure of just those users who use the Alexa toolbar?

One place to check is Wikipedia, which recognizes the same challenge when it reports: "The other "true" search engines (those that provide their own results), like Gigablast, have vastly less market presence than the big three. However, since site usage is proprietary information, it is often difficult to determine which sites are most popular."

Gigablast? Am I the only one who hasn't heard of that?

There are ridiculously expensive research reports we could buy, but fortunately a bit of digging with the leader in the search space, Google reveals some interesting statistics.

Market research firm Hitwise reports the following: "google.com accounted for 60.94% of all searches, search.yahoo.com accounted for 22.34%, search.msn.com was good for 10.55% and www.ask.com snuck into the top four with 4.34% of user searches for the month of October, 2006."

Keynote research echoes this research: "For the fourth consecutive wave of the Keynote study, Google topped the Keynote Customer Experience Rankings, an aggregate ranking of leading search engines based on an analysis of 250+ metrics measured during the study. In fact, Google outperformed its competitors in all 13 business success drivers measured in the study, including those for general search quality, local search quality and image search quality. Yahoo! Search ranked second in the study, Ask Jeeves third, MSN fourth and AOL's public site ranked fifth."

Finally, I'll let my colleague John Battelle have the final word, because his observation is so darn interesting: "the heaviest users of search, who are a minority of total search users, account for the vast majority of search queries. This seems a case where the tail is not as powerful as the head....And clearly, the more folks use the web, the more they use search."

Not exactly what you're asking, but I can't find one definitive source for ranking the top twenty or even top ten search engines by usage. Further, there are so many that are specialized for research, developers, scientists, or by vertical that the real question is which are the most popular search engines for your marketplace anyway.

Nonetheless, there are doubtless resources I've missed here. Anyone want to share their own statistical findings on this subject?