Google Inc. is challenging Facebook Inc. by using a controversial tactic: requiring people to use the Google+ social network.

The result is that people who create an account to use Gmail, YouTube and other Google services—including the Zagat restaurant-review website—are also being set up with public Google+ pages that can be viewed by anyone online. Google+ is a Facebook rival and one of the company's most important recent initiatives as it tries to snag more online advertising dollars.

The impetus comes from the top. Google Chief Executive Larry Page has sought more aggressive measures to get people to use Google+, two people familiar with the matter say. Google created Google+ in large part to prevent Facebook from dominating the social-networking business.

Both Facebook and Google make the vast bulk of their revenue from selling ads. But Facebook has something Google wants: Facebook can tie people's online activities to their real names, and it also knows who those people's friends are. Marketers say Google has told them that closer integration of Google+ across its many properties will allow Google to obtain this kind of information and target people with more relevant (and therefore, more profitable) ads.

Some users of Google's services are startled to learn how far the integration can reach. Sam Ford, a 26-year-old Navy petty officer, says he signed up for Google+ on his smartphone because it would let him automatically upload new photos to a Google+ folder—one that he kept private. Later, he says, he was surprised to see that his Google+ profile page—which includes his name—was tied to a software review that he wrote recently on the Google Play online store.