In a letter to Facebook Chief Executive Officer Mark Zuckerberg dated April 27, Senators Charles Schumer, Michael Bennet, Mark Begich and Al Franken objected to changes that made a user's current city, hometown, likes, interests and friends publicly available, where they were previously only seen by friends.
They protested changes that would allow Facebook's third-party advertisers to store users' data for more than 24 hours, and a Facebook "like" button on websites such as CNN.com, Pandora and ESPN to share the information with Facebook friends.
"We are concerned ... that this feature will now allow certain third-party partners to have access not only to a user's publicly available profile information, but also to the user's friend list and that publicly available information about those friends," the senators' wrote.
Facebook has about 400 million users.
The senators said the Federal Trade Commission (FTC) would likely examine the issue.
"In the meantime, we believe Facebook can take swift and productive steps to alleviate the concerns of its users," they wrote. "Providing opt-in mechanisms for information sharing instead of expecting users to go through long and complicated opt-out processes is a critical step."
Google, Facebook and other websites depend on access to information about users to earn advertising revenue. The FTC has been under pressure to curb the use of private information.
Facebook released a reply to the senators from Elliot Schrage, a Facebook vice president, also dated April 27, in which he wrote, "These new products and features are designed to enhance personalization and promote social activity across the Internet while continuing to give users unprecedented control over what information they share."
Schumer also wrote to the FTC, asking it to create guidelines on the use and distribution of private information by online social networking sites like Facebook, MySpace and Twitter.
"These sites provide a valuable service to users by keeping them connected with friends and family and reconnecting them with long-lost friends and colleagues," Schumer wrote. "But the growth of these sites over the last several years demands we provide guidelines on how private information submitted by users is shared and disseminated."
FTC spokeswoman Claudia Bourne Farrell said in a telephone call that the agency planned "to develop a framework that social networks and others will use to guide their data collection, use and sharing practices."
(Reporting by Diane Bartz; Editing by Lisa Von Ahn)