While Google doesn't yet have 100% market share, it does not have more than 50% share of all search engine use. That means it has a larger audience than all other search engines combined! Google also happens to have the most complex algorithm and so any site that can crack the first page of Google will likely do equally well on Yahoo!,Microsoft, and the many other search engines. With this in mind, you should focus on Google as themain priority for SEO campaign.