This infographic was recently published by Search Engine Journal, and does a wonderful job of illustrating the current U.S. search engine market distribution. When most people talk to me about SEO for the first time, it seems like all they care about is Google. Obviously, there's some pretty good reasoning behind this. Most purchases anymore seem to come from online search, and Google is the leader in this area for right now. I strongly believe, however, that a modern SEO campaign needs to be well-rounded, taking all major search engines into account.
Now, nevermind that Yahoo! is currently the most popular default search page in Japan, and that Yandex currently controls the majority of search in Russia and most former USSR nations. Even in the United States, a Google-only approach forgets about 1/3 of the search market. With Bing continuing to show gains like the ones illustrated in the following graphic, it's no wonder that some copies of FireFox are now being packaged with Bing as their default search engine. They did what Google has only talked about for ages when they reworked the search engine results page, and as standards like schema.org progress, consideration for all of the new ways that your site can be displayed in these engines can no longer be ignored.