Google has made a fortune selling advertisements; now it is trying to put its hundreds of millions of users to work as company spruikers, using the profiles, pictures and recommendations of ordinary people to endorse products and services on the internet.
After the policy takes effect on November 11, users who review a video on YouTube, or a restaurant, could see their name, photograph and comments in ads on any of the 2 million websites that are part of Google's display ad network.
The controversial practice, announced at the weekend, is part of an emerging trend on the internet. Advertisers believe that consumers put enormous value on product endorsements by a friend or family member, and growing numbers of internet companies are trying to capture that social advertising in a systematic way.
Critics say tactics that further exploit the data people leave online amount to a bait-and-switch manoeuvre. People sign up for Google's services because they are free and convenient and they probably never thought their words and identities would be used to sell products to strangers.
Users who casually endorse a product or a song on Facebook or Google "may be exposed to unwanted implications", Santa Clara University internet law expert Eric Goldman said.
Google says "shared endorsements" will help consumers make better choices. "We want to give you - and your friends and connections - the most useful information. Recommendations from people you know can really help," the company said in its announcement.
It said users can opt out of the ads and that it will automatically exclude anyone under 18.
Facebook offers a similar advertising feature called "sponsored stories", which turns a recommendation using the "like" button into an advertising endorsement on a friend's Facebook page. The company has said users cannot opt out of the practice. About 1.2 billion people use Facebook.
Last month, the US Federal Trade Commission said it would review whether Facebook's push into sponsored stories violated the company's privacy settlement with the government in 2011. That agreement required Facebook to give adequate notice of changes in its privacy policies and to ensure users were not misled about how their data is being used.
Due to the US government shutdown, the commission said it could not respond to a question on whether its investigators would examine Google's new advertising practice.
Google said its new policy would apply only to the 390 million people who have signed up for its social network Google Plus.
The company can also draw on endorsements made with Google's +1 button, which is similar to Facebook's "like" button and appears on internet sites.
Users could limit the reach of their advertising endorsements so that reviews were shared with only a small circle of friends on Google Plus, the company said.
Some privacy experts have commended the way Google is rolling out the feature by giving users a month's notice of the changes and options to decline.
"Some people may like the fact that their reviews will be promoted. Others have a pretty easy way to opt out," a Future of Privacy Forum spokesman said.