A change to Facebook's terms of service, which extends control over user content to profiles that have been shut down, has prompted nearly 20,000 users to join protest groups.

Users signing a contract with Facebook give full control of their social networking content to the company, including the right to copy, modify, translate and distribute any of their information, and to use images and descriptions of themselves for any purpose, including commercial use and advertising.

Facebook used to relinquish control over content when contracts were terminated, but the latest change to its terms technically allows the site to keep the information forever.

Facebook chief executive Mark Zuckerberg posted a blog entry on Monday in an attempt to explain the new terms after facing unprecedented criticism from the media and thousands of users. "We wouldn't share your information in a way you wouldn't want," he said.

Zuckerberg explained that the change was designed to reassure users that account deactivation would not mean that data they had shared with friends would be wiped from the site.

"When a person shares something like a message with a friend, two copies of that information are created - one in the person's sent messages box and the other in their friend's inbox," he explained.

However, users are anxious that the terms allow Facebook to keep all their information, even in circumstances when it is not shared with others and is just posted as an interest or status update.

Facebook released a separate statement to provide more clarity. "If you send a message to another user (or post to their wall, etc), that content might not be removed by Facebook if you delete your account (but can be deleted by your friend)," said a spokesman.

Users have argued that the statement should be included in the contract terms. A group on the site called 'Facebook Owns You', which has nearly 2,000 members, said: "This is great, but it really needs to be in the TOS [terms of service] - otherwise it's just fluff talk."

Another Facebook group called 'People Against the New Terms of Service', which has 16,427 members, called for the terms to be changed to how they were before the update on 4 February.

A member from 'Those Against Facebook's New TOS', which has nearly 700 members, said: "I'm assuming they didn't have a problem letting users have control over deletion of their own data before (being that the TOS reflected as much at that point), so what's changed? If I decide to close my account and have all associated data deleted, that should be my choice. If I want to yank a blog post or a tweet, I have that ability. Why should Facebook be any different?"

A UK spokesman for Facebook said that it is unlikely that the company would include the statement in the formal terms and conditions.

Facebook's unwillingness to respond to user demands is not unusual. The firm declined to respond after receiving more than 2.6 million complaints about the new layout of the site, and a group called 'Reform the Facebook Privacy Policy' has been campaigning since June 2007 for Facebook to stop sharing information on its users with companies, lawyers, agents and government agencies.