Google started including "your social circle" in its search results earlier this year. Ever wonder how Google knows who you know? Wonder no more, as the Mountain View firm offers a page explaining exactly how interconnected your online life really is.
The link below leads you to a page where Google explains the three levels of contact it can trace between you and other people, with the depth depending on whether you've filled out a Google Profile and how busy you are on Google services like Chat and Reader. You'll see your "direct connections" through Chat and other contact-creating apps, direct connections from sites you've linked to in your profile (including those you follow on services like Twitter), and those friends-of-a-friend through your direct connections.
On the one hand, it's just a mind-boggling or slightly scary scroll to see how many people Google can connect you with, depending on your temperament. On the other, it can be handy in tracking down all the ways you can get in touch with somebody beyond the one address you have for them — those contacts who have Google Profiles are linked in this list, and you can always send direct messages through other mediums as well.