As a protective measure, Facebook will warn minors opting to be more open that they are exposing themselves to a broader audience. The caution will repeat before every post, as long as the settings remain on "public."
The initial privacy settings of teens under 18 will automatically be set so posts are seen only by friends. That's more restrictive than the previous default setting that allowed teens to distribute their posts to friends of their friends in the network.
In a blog post, Facebook said it decided to revise its privacy rules to make its service more enjoyable for teens and to provide them with a more powerful megaphone when they believe they have an important point to make or a cause to support.
"Teens are among the savviest people using social media, and whether it comes to civic engagement, activism, or their thoughts on a new movie, they want to be heard," Facebook wrote.
The question remains whether teens understand how sharing their thoughts or pictures of their activities can come back to haunt them, said Kathryn Montgomery, an American University professor of communications who has written a book about how the Internet affects children.
"On the one hand, you want to encourage kids to participate in the digital world, but they are not always very wise about how they do it," she said. "Teens tend to take more risks and don't always understand the consequences of their behavior."
The relaxed standards also may spur teens to spend more time on Facebook instead of other services, such as Snapchat, that are becoming more popular hangouts among younger people. Facebook CEO Mark Zuckerberg, though, says that the company's internal data shows its social network remains a magnet for teens.
Giving people more reasons to habitually visit its social network is important to Facebook because a larger audience helps sell more of the ads that generate most of the Menlo Park, Calif., company's revenue.
"What this is really about is maximizing the kind of sharing at the heart of Facebook's business model," Montgomery said. She worries that unleashing teens to share more about themselves to a general audience will enable advertisers to collect more personal data about minors "who aren't aware that their movements and interests are under a digital microscope."
Facebook hasn't disclosed how many of its nearly 1.2 billon users are teens. The social network was initially limited to college students when Zuckerberg started it in 2004, but he opened the service to a broader audience within a few years.
The teen audience is large enough to give Facebook periodic headaches. As its social network has steadily expanded, Facebook has had to combat sexual predators and bullies who prey upon children.
Facebook doesn't allow children under 13 to set up accounts on its service but doesn't have a reliable way to verify users' ages.