Four U.S. lawmakers expressed concern to Facebook on Tuesday about changes in its privacy policy, while one asked federal regulators to draw up privacy guidelines for online social networking sites.

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 also protested changes that would allow Facebook's third-party advertisers to store users' data for more than 24 hours.

They expressed concern about changes that allow members to click on a Facebook like button on websites such as, Pandora and ESPN, thus sharing the information with Facebook friends.

We are concerned, however, 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' letter said.

Facebook has about 400 million users.

In their letter to Facebook Chief Executive Officer Mark Zuckerberg, the lawmakers warned that the Federal Trade Commission was likely to look at the issue.

In the meantime, we believe Facebook can take swift and productive steps to alleviate the concerns of its users, the senators 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 a host of websites depend on access to information about users to earn advertising revenue and keep the services free. But the FTC has been under pressure to rein in the use of what many consider private information.

Facebook released a copy of a letter that it sent to Schumer, in which it argued that its users have complete control over what information gets shared with whom.

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, wrote Elliot Schrage, a Facebook vice president.

Schumer also wrote a letter to the FTC, asking the agency to draw up 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, wrote Schumer, but the growth of these sites over the last several years demand we provide guidelines on how private information submitted by users is shared and disseminated.

All three letters were dated April 27.

The FTC had no immediate comment.

(Reporting by Diane Bartz; Editing by Lisa Von Ahn)