Google Social Search is now "following a more comprehensive set of public links from your Google profile to populate your "social circle" and to find social content. If you're signed-in, Google makes a best guess about whose public content you may want to see in your search results -- the content you see comes from your "social circle."
Here's how it works: If you Buzz, you've option to add "Connected Sites" and post content from these sites publicly on your Google profile. For e.g., you can choose to connect your Twitter account and set your tweets to appear publicly in your Buzz stream and Buzz tab of your profile," noted Google.
If you don't use Buzz, you can still add links to YouTube, Picasa and other sites directly to your Google profile, and we'll continue to follow those links as well.
You can see exactly what content you're sharing and who you're connected with by clicking "My Social Content" and "My Social Circle" next to any social search result. As always, you can click "more search tools" and select "social" to see more results from your network.