If you have a Google profile, depending on how much information you have filled in and how active you are on the internet, Google knows who you know.
Click here to find out.

[It] is the network of connections Google uses to identify relevant social search results. It is based on a combination of the following:

• Direct connections from your Google chat buddies and contacts
• Direct connections from links that appear on your Google profile

• Secondary connections that are publicly associated with your direct connections

In addition to web pages from your social circle, posts from your Google Reader subscriptions may also appear in your social search results.

via boingboing.net