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We're rolling out a new experiment on Google Labs called Google Social Search that helps you find more relevant public content from your broader social circle. It should be available for everyone to try by the end of the day, so be sure to check back. (...) With Social Search, Google finds relevant public content from your friends and contacts and highlights it for you at the bottom of your search results. When I do a simple query for [new york], Google Social Search includes my friend's blog on the results page under the heading "Results from people in your social circle for New York." I can also filter my results to see only content from my social circle by clicking "Show options" on the results page and clicking "Social."

http://googleblog.blogspot.com/2009/10/introducing-google-social-search-i.html