If you'd like to participate, simply go to the Google Labs page (http://www.google.com/experimental/). Look for the Social Search experiment and click "Join This Experiment." You can only participate in one experiment at a time, so you'll have to leave any other Google Labs experiments you are a part of.
Then, go to Google and conduct a search. Google will use networks from your Google Profile to deliver social search results, if there are any. You'll see them in their own section at the bottom of the results. The results only include results from your network.
You'll need to let Google know about your networks in your profile if you haven't yet done so. Simply go to your Google account settings, click "Edit Profile" and enter in your Twitter, Facebook, YouTube, etc. You can put as many or as few as you like.
For more information on Google's Social Search experiment, check out these videos: