It looks like you can mostly expect this to work with foks who signed on to OpenSocial. As the article points out: don’t expect it to work with Facebook.
There’s a new Google product called “Social Search” that is launching soon in Google Labs. This is a new feature that allows you to see results for queries from people in your social network. This works by using your Google Profile. If you fill it out with the other social networks you’re a member of, such as FriendFeed, Google will scan who you are connected to and give your results from those people.