Facebook has quietly changed its terms of service. Formerly, if you canceled your account, all your content was deleted and that was that. Under the new terms, Facebook retains an "irrevocable, perpetual" license allowing the company to do whatever it likes with whatever you have posted.
The change is likely an indication that Facebook ad revenue is not paying all the bills, and they want the ability to harvest email addresses and contact information and resell the data forever, basically.
While this is an issue in its own right, another issue is the potential for legal discovery to create problems for people years or decades later. For example, someone in their forties who is being sued might find out the opposing attorney has subpoenaed data from a Facebook or Google account that you had closed out twenty years ago. There is no "free" in these "free" services, and these kinds of service agreements continue to underscore the need for local services that have some degree of community control over the terms of use and content. Facebook is an interesting social networking tool with many good things in its favor, but the buyer must beware.
Updated....
Today (2/18/2009) Facebook reversed course and has reverted to the previous Terms Of Service (TOS). There was a huge online backlash against the proposed changes. Kudos to Facebook for recognizing a mistake and fixing it.