This development comes on top of a lot of other personal data collection at Google - and those things have me much more concerned about privacy abuse that this one does.
For this type of social search to kick, in you need to be intentionally opted in through many layers.
First, you need to be logged into your Google account.
Second, you need to have set up a Google profile.
Finally, that profile needs to register your other social media profiles with Google.
No one who has jumped through those hoops can say that they didn't know they were giving data to Google. You might as well volunteer to have an RFID chip implanted in your skull, and then cry "I didn't know".