Google pulls publicly available Web content from a person's social circle through blog posts, status updates, tweets and pictures. The set of connections, or social circle, provides the network. Google says it's using a normal ranking algorithm to pick the content.
There are three ways that Google pulls content to build a social graph. The search engine will source social content through public Google profiles and extended profiles, Google Chat buddies, and content in the Google Reader.
Google's Search Guru Matt Cutts explains how the engine creates the social circle in a video on YouTube. "If I follow my best friend, and he's following five people, Google will add those five people to my extended social circle since it's likely some of those connections will be mine as well," he says. "A second way for us to add to your social circle is from your Google Chat buddies. If you have eight Chat buddies, we will add those eight Chat buddies to your social graph."
Cutts says Google will not publish the list of Chat buddies. Google pulls them into the person's social circle so the engine can serve up their public information, too. The third way is to capture information through Google Reader.
Subscribing to a blog in Google Reader allows Google to serve a post from the blog in search results.
After the social circle is built, the information serves up in Google search query results at the bottom of the page when its algorithms determine the content will benefit the person searching.
The social content, however, only serves up when signed into Google through Google Labs. Cutts says people have control of the content, and that things that are not public do not show up in the search results.