Opt out / more evil
You can apparently disable this - or "opt out of this social experiment". According to
"To opt out, visit http://www.google.com/experimental and click the Leave button for the experiment." Although it doesn't explain exactly what opting out means - will you still show up in other people's social searches?
Rather more spookily, the page also says "If someone you don't know shows up in your social search results, it's likely that they're connected to someone you do know." So not only are they looking up your friends, they're linking you to your friends' friends, or your acquaintances' boss/parents/ex.