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Try searching for any term on google (while signed in) and you’ll see results from your Social circle toward the end of the page.
Google knows your social contacts from you gmail address book and searches for content created by your friends.
From Google: If you’re signed in to Google, you might see social search results for a particular search at the bottom of the results page under “Results from people in your social circle.” All the content shown within Social Search is publicly-available online through Google and other search engines. Social Search highlights content from your social circle to provide a personalized search experience.
People in your Gmail contacts, people you are connected on publically viewable social networks like Twitter/Friendfeed, people you might be following on Google Reader/Buzz and People who are connections of those in your immediate public social circle may be the ones who Google searches the content from.