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If it isn't in Google, it probably doesn't exist.
That statement may strike you as an exaggeration, but a lot of people act as if it were true. They run a quick Google search, and if they don't find what they're after right away, they're more likely to adjust their query than move on to another search engine. So if your site is listed on AltaVista, Yahoo, and every other search index on the planet, but Google doesn't know about it, chances are good that millions of people are going to miss it, too.