|© 2011 Crossbridge Communications, LLC|
In the US, its share tends to hover in the 60% to 70% range, with July 2011 results reported as 65.5%. Globally, Google dominates even more dramatically, controlling 83% of the market. 
To offset the differential in size, Microsoft and Yahoo formed an alliance of two Davids to challenge the giant known as Google.
This search alliance was formed two years ago and has been in effect in the US for about a year.
Basically the alliance means Yahoo is using Microsoft’s Bing technology and algorithms, and together they’re striving to create a “new unified search marketplace.” 
Has it made a difference?
As of July 2011, after years of pouring billions into the effort, Bing’s market share is up 3% and it now controls 14.1% of the search pie. Meanwhile, Yahoo has dropped from 18.9% to 15.9%. In combination, they control 33% of the market. 
How does this affect you?
Competing search engines with different search algorithms produce different results. The simplest way to prove this point is to conduct a quick search for your organization name through each of the big three and compare the results. (You should routinely do this anyway.)
It's also important to decide just how much time, energy, effort and money you and your organization are willing to invest in SEO. That challenge has become a bit simpler, with the alliance of Bing and Yahoo.
Whether you're budgeting dollars or time, two strategies make sense. One is to split your investments to roughly mirror relative market shares (33% v. 66%). The other is to focus exclusively on the dominant player.
Right now, that's Google.
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1. Bing Increases Market Share, But Google Still Dominant
2. Yahoo-Microsoft Search Alliance