Days after its younger, larger competitor Facebook came under attack for sharing unwitting users' data with ad agencies and Internet tracking companies, it has been discovered that MySpace also has been disclosing similar information with third parties.

The information was usually sent by MySpace when users clicked on advertisements, according to the Wall Street Journal, which first discovered the ad tracking. Like the Facebook disclosures, advertisers potentially can tap the user IDs to find public information such as full names, locations, ages, and phone numbers. MySpace advertisers -- such as Google, Quantcast, and Rubicon Project -- reportedly said they did not use the information they received.

"Knowledge of a public user ID does not give anyone access to private user data. We share non-personally identifiable information with advertising companies as part of our ad serving process," a MySpace spokesman told CNN. "It has recently come to our attention that several third-party app developers may have violated these terms and we are taking appropriate action against those developers."

Unlike Facebook, which under its terms and conditions can close accounts found to use fake names, MySpace users can create accounts under pseudonyms, and can include false information without penalty. Since a MySpace user's profile name appears in the URL of the page the person is viewing, whenever the accountholder clicks on an ad, app, or game, an advertiser easily can see which page -- account and user ID -- the individual came from.

Some MySpace apps transmitted user IDs. For example, BitRhymes' TagMe, with 8.3 million users; WonderHill's 1.8 million-user GreenSpot; and RockYou's RockYou Pets with 6.1 million users, transmitted IDs, according to the Journal. News Corp. owns MySpace and the Journal.

"[BitRhymes] has a strict policy of not passing personally identifiable information to any third parties. When we were informed of the issue, any suspect relationship was immediately dissolved," the company said in a published statement.

A company that works with RockYou was transmitting user information to a third company without RockYou's knowledge, a company spokeswoman said in a published report. "We have taken immediate action to indefinitely suspend their services in connection with RockYou and we are reviewing all third-party providers to ensure compliance with our platform partners' terms of service," she told the Journal.

Facebook had 148 million visitors in September, according to ComScore, compared with the 58 million who visited MySpace.

Facebook had blocked one game developer -- Lolapps -- but then restored access later that weekend. Two congressmen have urged Facebook to explain how apps could share Facebook members' user ID numbers, and how the company planned to prevent this from reoccurring in the future.