Google Social Search is an experiment on Google Labs. To use it, you need to switch it on from the Google Experimental page. You also need a public Google Profile. Once you have one, you can add your social networks to your profile like I have done in my Google profile.
Google then proceeds to index your profile and the links to your social network. If the thought of his freaks you out (like Dan Costa at PCMag), rest assured that the whole process is opt-in.
If you choose to opt in, Google will index the networks you have added to your profile — your friends and their friends for an extended social circle, as Google calls it.