SAN FRANCISCO -- Do you know who can see what you are posting on Facebook, including your photos, birthday and personal cellphone number?

Chances are that you don’t.

Responding to business pressures and longstanding concerns that its privacy settings are too complicated, Facebook announced last week that it was giving a privacy checkup to every one of its 1.28 billion users.

The company, which is based in Menlo Park, Calif., will also change how it treats new users by initially setting their posts to be seen only by friends. It will explain to them that adjusting the setting to "public" means that anyone on the Internet can see their photos and messages.

The change in default settings and the person-by-person review, which may shock users who suddenly realize how widely their personal information has been shared, is a sharp reversal for Facebook.

"They have gotten enough privacy black eyes at this point that I tend to believe that they realized they have to take care of consumers a lot better," said Pam Dixon, executive director of the World Privacy Forum, a nonprofit research and advocacy group. Dixon was briefed in advance about the latest changes.

For most of its 10-year history, Facebook has pushed -- and sometimes forced -- its users to share more information more publicly, drawing fire from customers, regulators and privacy advocates across the globe.

But for Mark Zuckerberg, the company’s co-founder and chief executive, more sensitivity to privacy might be good business.

Zuckerberg has seen privacy-friendly services like WhatsApp and Snapchat, and anonymous-sharing apps like Secret and Whisper, emerge as a competitive threat, particularly among younger users. That prompted him to strike a deal earlier this year to buy WhatsApp for as much as $19 billion.

"What we really want is to enable people to share what they want," Zuckerberg said in an interview last month. "People read a lot of the stuff that we do as if we are trying to somehow get people to share more things, but all the core innovations are around giving people the tools they need to be comfortable."

Facebook might also be acting to forestall any actions by regulators, who constantly scrutinize its privacy practices. European privacy officials are reviewing the company’s proposed acquisition of WhatsApp, which follows stricter privacy procedures than Facebook does. In the United States, the company is wrangling in a federal appeals court over how it uses the personal data of teenagers in advertising sent to their friends.

Even as Facebook takes steps to empower its users on privacy, it continues to introduce features that raise new issues. Last week, it announced an optional service for mobile phones that eavesdrops on the sounds in a room to try to identify any music or television shows that might be playing. Facebook says it doesn’t store the information for long, but it is the first time the company has tried to listen in on its users’ lives.

But Facebook has also made several other moves recently that indicate that it is taking privacy more seriously. Last month, it began a location-sharing feature called Nearby Friends that is entirely optional and provides only a user’s general location. And it made changes to Facebook Login, a service that allows people to use their Facebook identities to log in to other sites and apps, that reduce the amount of information shared outside Facebook, and in some cases allow people to share nothing at all.

In its announcement, Facebook said that when people sign in to post something on the company’s popular social network, they will soon see a cartoon blue dinosaur that pops up with the message, "We just wanted to make sure you’re sharing with the right people."

The service will then walk users through the privacy settings for their status updates, remind them of the applications that have permission to use their Facebook data, and review the privacy settings for some of the most private information on their profiles, such as their hometown, employer, email address, phone number and birth date.

"People sometimes feel that their information is more public than they want," Mike Nowak, a Facebook product manager who worked on the privacy checkup, said in an interview. "It’s not fun when you share something, and someone you didn’t expect to be able to see it can see it."