Facebook is trying to become a bit more like the real world.
The company on Wednesday introduced a feature that allows users to interact with small groups of people, like their family, high school friends or colleagues.
The move is an effort to address a longstanding problem: Facebook friends often span a broad range of relationships that include relatives, classmates, casual professional acquaintances or jogging partners -- and not everyone wants all of them to see his or her information.
With the new feature, called Groups, Facebook hopes to encourage users to upload more photos, videos and other information to the site while giving them new ways to control who sees what.
Some privacy advocates welcomed Groups, but others worried that it would give Facebook even more information about users, which it could provide to marketers and others.
In an interview, Mark Zuckerberg, co-founder and chief executive of Facebook, said the feature, which has been in development for months, was not created in reaction to the recent series of privacy mishaps that have forced the company to roll back new features and apologize to users.
"We think about this stuff a lot," Mr. Zuckerberg said, referring to privacy. "Often people don't think we think about it as much as we do."
He said Groups was simple in concept but technically difficult. At a news conference at Facebook headquarters here, he noted that Groups was not the company's first effort to let users share more selectively.
Facebook has long allowed users to create lists of friends, but that feature has largely failed. Mr. Zuckerberg said only about 5 percent of Facebook users take advantage of the lists.
Groups allows anyone to create a group and include other people. For example, someone's cousin may create a group for their family and put every family member in it. In that way, Facebook contends that if even a small percentage of users create groups, most people on Facebook will end up in several groups.
"We think this is going to be a pretty fundamental shift for how people use Facebook," Mr. Zuckerberg said. "The amount of sharing will go up massively and will be completely additive."
Once a group is created, users can upload information only to that group, and can communicate with everyone in the group simultaneously through online chat.
Mr. Zuckerberg said that other applications and services that use Facebook's technology would be able to use Groups, and that Groups would help improve other parts of Facebook.
"Knowing the groups you are part of helps us understand the people who are most important to you, and that can help us rank items in the news feed," he said.
Augie Ray, an analyst with Forrester Research, called Groups "a big step forward to allow Facebook to reflect the same social norms that we have in the real world."
He added: "In the real world, we don't shout one thing to everyone that we know. With Groups, you have this opportunity to begin to control who hears what."
Larry Magid, co-director of ConnectSafely.org, a nonprofit group focused on online safety for children, said Groups could protect privacy. "But there is the danger of a false sense of security," said Mr. Magid, whose organization receives some financial support from Facebook and other technology companies. Mr. Magid said a group member could deliberately or inadvertently allow a stranger into a group, potentially exposing information in the group to an outsider.
Mr. Zuckerberg said he thought the system would police itself because everyone in the group would be notified when a new member joins and would flag someone who does not belong.
But Marc Rotenberg, executive director of the Electronic Privacy Information Center, called the new service "double-edged."
"Yes, it's good to be able to segment posts for particular friends," he said. "But you will also be revealing information to Facebook about the basis of your online connections."
Mr. Rotenberg said he worried about how Facebook would use that information.
On Wednesday Facebook also introduced two other services: one to allow users to download all their Facebook information into one file, and another to let people monitor how third-party applications gain access to their personal data.