If you do not have to pay for a service, you are not the customer, you are the product being sold.

Published: October 11, 2013

SAN FRANCISCO — Google, following in Facebook’s footsteps, wants to sell users’ endorsements to marketers to help them hawk their wares.

On Friday, Google announced an update to its terms of service that allows the company to include adult users’ names, photos and comments in ads shown across the Web, based on ratings, reviews and posts they have made on Google Plus and other Google services like YouTube.

When the new ad policy goes live Nov. 11, Google will be able to show what the company calls shared endorsements on Google sites and across the Web, on the more than two million sites in Google’s display advertising network, which are viewed by an estimated one billion people.

If a user follows a bakery on Google Plus or gives an album four stars on the Google Play music service, for instance, that person’s name, photo and endorsement could show up in ads for that bakery or album.

Google said it would give users the chance to opt out of being included in the new endorsements, and people under the age of 18 will automatically be excluded.

Such product endorsements, especially coming from friends and acquaintances, are a powerful lure to brands, replicating word-of-mouth marketing on a broad scale.

But as Facebook has learned, many users have strong and skeptical feelings about their endorsements being used in ads without their explicit permission.

“The trick to any advertising like this is to avoid coming across as creepy to your user base and have them say, ‘I didn’t want anyone else to know that,'” said Zachary Reiss-Davis, a Forrester analyst, speaking generally about social ads.

In a notice to users posted on its site on Friday, Google said, “Feedback from people you know can save you time and improve results for you and your friends across all Google services.”

Facebook, the world’s largest social network with 1.2 billion users worldwide, has been aggressively marketing such social endorsements. For example, if you post that you love McDonald’s new Mighty Wings on the chain’s Facebook page, McDonald’s could pay to broadcast your kind words to all your friends, effectively using you as a product endorser.

The company declined to specify exactly how it planned to use endorsements in advertising, what the ads would look like or how brands choose whether to include shared endorsements.

Facebook does not allow its users to opt out of such ads, which it calls sponsored stories, although users can limit how their actions on the social network are used in some other types of advertising.

Google Plus users, on the other hand, will be able to opt out of inclusion in ads on the social network’s settings page.

If a Google Plus user has shared comments with a limited set of people, only people in that circle will see the personalized ads. Ratings and reviews on services like Google Plus Local are automatically public and can be used in ads, unless a user opts out of shared endorsements.

Google had previously shown so-called +1s, votes of approval similar to Facebook likes, in ads across Google sites and its ad network. Google plans to expand that to include “follows,” comments, ratings, reviews and other interactions. Those who have already elected to opt out of using +1s in ads will automatically be opted out of the expansion.

Though 190 million users post on Google Plus and 390 million use the social network indirectly by sharing on other Google sites like YouTube, Google’s variety of services gives it a potentially wider reach.

Currently, Google does not have an ad unit incorporating more social data ready to be used by advertisers, the company said. Instead, the company wants the ability to create such an ad unit in the future and is notifying users in advance.

Although advertising irks some users — even while it helps support free services — social ads have proved particularly contentious.

Facebook recently settled a class-action lawsuit that claimed it had not adequately notified users about how it was using endorsements. In late August, it tried to impose a new privacy policy that would have given the company clearer rights to run social ads without a user’s explicit permission. After privacy groups complained, the Federal Trade Commission began an inquiry into the changes, prompting Facebook to suspend the process.

Google, which is under the supervision of the F.T.C. for a previous privacy violation and has agreed to privacy audits and fines for privacy misrepresentations, is taking pains to show that it has considered the privacy implications of the new ads.

It will notify users of the change with banners on Google’s home page, in search results, in Google Plus notifications and elsewhere. And posts by users who have registered as being under age 18 will not appear in ads, though their posts can still appear in search results or other places that are not commercial in nature.

Shared endorsements are the latest example of the continual push by Google and other Web companies to collate in one place the reams of personal information people share online and use it to personalize people’s online experiences.

Privacy advocates say companies do not generally get meaningful consent from their users before using such information.

“Users reasonably expect that their comments should be used as they intended," said Marc Rotenberg, executive director of the Electronic Privacy Information Center, which has tangled with numerous Internet companies, most recently Facebook, over the use of personal information in ads. “People don’t typically race around handing their friends leaflets and advertisements.”