(Bloomberg) — US Senate Commerce Committee Chairman Jay Rockefeller pressed executives of Facebook Inc. and MySpace Inc. for information about breaches involving personal information and vowed to write legislation protecting privacy.

Rockefeller, a West Virginia Democrat, said he is troubled by reports that users of the sites had personal details transferred to third-parties without their knowledge. "I intend to find out whether today's social-networking sites are adequately protecting their users' personal information," Rockefeller said today in a statement. "I fully intend to conduct oversight and formulate strong public policy that protects the privacy of American consumers."

Facebook is under pressure to protect customer information as it seeks revenue from the 500 million users who play games, post photos and communicate using the site.

The internet company said October 18 it's taking steps to block information that can identify customers from being passed to advertisers and outside companies, a day after the Wall Street Journal reported the breaches.

Games and applications that are popular on Facebook collect user information which is turned over to the networks that handle ad placement online and to companies tracking Web surfing. Such software passed along Facebook ID numbers, which may be used to obtain customer names, in a way that violated company policy, Facebook said at the time.

Rockefeller wrote Facebook founder and Chief Executive Officer Mark Zuckerberg and MySpace President Michael Jones seeking details about the privacy policies and enforcement.

"The privacy of tens of millions of Facebook users could be seriously compromised in violation of the company's stated policy," Rockefeller wrote to Zuckerberg. The senator asked how the company enforces its privacy policy, what penalties it imposes for violations and what steps are taken to avoid future breaches.

"The issue is caused by an inherent weakness in Web browsers, which automatically share information with visited websites," said Andrew Noyes, Facebook's spokesman in Washington.

He said Facebook had decided to encrypt user IDs and will work with Internet browser makers to resolve the issue. Facebook will respond to Rockefeller's questions, he added.

The policy at MySpace, which is owned by News Corp., raises "serious questions about your commitment to develop and maintain strong privacy protections for consumers," Rockefeller wrote to MySpace's Michael Jones.

MySpace is reviewing the letter and will "respond in a timely manner," Rosabel Tao, a spokeswoman for the Beverly Hills, California-based company, today in an e-mail.

Representatives Edward Markey, a Massachusetts Democrat, and Joe Barton, a Texas Republican, wrote a letter to Zuckerberg on October 19, seeking details on who likely has been affected and how the company aims to change its policies.