Facebook has denied it monitors users who have logged out of the service. Australian hacker Nik Cubrilovic this week accused Facebook of using to cookies to track users even after they've logged out of the site.

Facebook engineer Gregg Stefancik denied that the company tracked users in an online post. Stefancik did say that Facebook alters cookies when the user logs out but that it doesn't use those cookies to track users or sell their personal information.

In a written statement, Facebook said: "Facebook does not track users across the web. Instead, we use cookies on social plug-ins to personalize content (e.g. show you what your friends liked), to help maintain and improve what we do (e.g. measure click-through rate), or for safety and security (e.g. keeping underage kids from trying to sign up with a different age). No information we receive when you see a social plug-in is used to target ads, we delete or anonyms this information within 90 days, and we never sell your information."

These cookies are there to protect consumers, the company said. They are used to identify spammers and phishers, detect unauthorized access and to help users regain access to account if it has been hacked. The cookies also disable registration for underage users who try to log-in using a different birthdate.

However critics are arguing that these cookies and some of Facebook's new changes are just more attacks on privacy.

Marc Rotenberg, the executive director of watchdog group Electronic Privacy Information Centre has sent a letter to the Federal Trade Commission about his organization's concerns about the social media site.

"It's getting really difficult to evaluate the changes that Facebook makes, and I say that as a privacy professional. I can't imagine what the typical user goes through," Rotenberg said. "Users might opt in to what Facebook is planning to do, but Facebook never gives users that option. It just marches forward and users have to go along."

An agency spokeswoman said the FTC did not discuss investigations unless the subject of an inquiry disclosed the investigation.

"Then we can confirm the investigation without providing any details. But Facebook has not done so, so I can neither confirm nor deny that the FTC is investigating Facebook," Claudia Farrell said in an email.