SAN FRANCISCO — Facebook is taking heat from U.S. lawmakers after reports that the company is exploring ways to let children under 13 onto its social network.

The co-chairmen of the Bi-Partisan Congressional Privacy Caucus, Reps. Joe Barton, R-Texas, and Edward Markey, D-Mass., sent a letter to Facebook Chief Executive Officer Mark Zuckerberg on Monday asking him to provide details on the company's plans for allowing access to children under age 13, who fall under stricter regulations around online privacy.

Facebook is looking at ways to let younger children use its website, though the company hasn't made a final decision on whether or how to give them access, said a person familiar with the matter who asked not to be identified because the discussions are private. While the move would expand Facebook's user base, it would also invite further scrutiny over privacy and security on the world's largest social network.

"We strongly believe that children and their personal information should not be viewed as a source of revenue," the representatives wrote in the letter. "We are deeply concerned that the changes discussed by Facebook could potentially have a harmful impact on our children."

Children 12 and younger are protected by the 1998 Children's Online Privacy Protection Act, which requires websites to notify parents and obtain their consent before collecting personal data from minors.

Complying with this law while opening Facebook's service to younger users could create technical challenges, said Jeff Chester, executive director of the Center for Digital Democracy, a Washington-based privacy group.

"Facebook can't simply get a parent to say OK and have children face a slew of viral marketing and ad campaigns," Chester said. "If Facebook is to serve children, they have to create a state-the-art system to protect their privacy."

Still, many children are already accessing the site, and Menlo Park, Calif.-based Facebook said in a statement that it's looking for ways to help keep young kids safe while on social networks.

In May 2011, Consumer Reports said a survey showed that Facebook had 7.5 million users who were younger than 13 in the prior year, in violation of the website's policies. More than 5 million of the users were under age 11, the watchdog group said.

"Many recent reports have highlighted just how difficult it is to enforce age restrictions on the Internet, especially when parents want their children to access online content and services," Facebook said in an e-mailed statement. "We are in continuous dialogue with stakeholders, regulators and other policymakers about how best to help parents keep their kids safe in an evolving online environment."

The company is seeking ways to expand its user base and find new sources of revenue as concerns mount about its sales growth. Since its initial public offering May 18, Facebook shares have tumbled 29 percent. The social network has swelled to more than 900 million members worldwide, yet growth in advertising revenue has failed to keep pace as more users access the site from mobile devices.

Facebook, already accused by regulators of failing to protect members' privacy, must be careful to shield young users and keep them safe online, said Rep. Mary Bono Mack, R-Calif.

"In developing this new technology, Facebook needs to proceed with an abundance of caution," said Bono Mack, who heads a House Energy and Commerce subcommittee that has looked at online privacy issues, in an e-mailed statement. "Very strict privacy protocols must be in place before younger children are allowed on social-networking sites. Before this happens, Facebook also has a responsibility to expand its efforts to educate parents and preteens about the dangers of cyberspace and how to use the Internet and social networking in a safe way."

The Wall Street Journal reported earlier that Facebook was developing technology to let younger children use its website under parental supervision.

Allowing children to access Facebook at a younger age could help the company educate a new generation of users about its service, much as Apple Inc. has introduced children to its product line through the iPod music player, said Colin Sebastian, an analyst at Robert W. Baird & Co. in San Francisco.

"Maybe there is a way to engage kids with a free service with no advertising, but with strict policies which protect children — that might be smart of them," Sebastian said.

Any attempt to court children at a young age to build loyalty with them is likely to meet still more criticism, said James Steyer, CEO of Common Sense Media.

"What Facebook is proposing is similar to the strategies used by Big Tobacco in appealing to young people: try to hook kids early, build your brand, and you have a customer for life," Steyer said in a statement. "What's next? Facebook for toddlers?"

— With assistance from Brian Womack in San Francisco. Editors: Jillian Ward, Lisa Rapaport, Stephen West