Google has an extensive user research team, and make extensive use of quantitative and qualitative feedback. They have scores of everyday users physically come in to the office or remotely participate in the studies every week.
Search for cars while you are opted-out of personal search. You will see the Google+ Ferrari page on the right. Is that really what people want to see? The normal Ferrari website isn't anywhere in the top 100 organic results, but yet Google's extensive user research team decided that is what people wanted to see on the right hand side. Or did they put that there because it benefits them.