Thanks. This is what they say:

  > each time you visit a third-party webpage that has a Facebook 
  > Like button... the unique characteristics of your PC and browser, 
  > such as your IP address, screen resolution, operating system and
  > browser version, are also recorded.

  > Bejar acknowledged that Facebook could learn where specific 
  > members go on the Web when they are logged off by matching the 
  > unique PC and browser characteristics.... He emphasized that 
  > Facebook makes it a point not to do this. "We've said that we 
  > don't do it, and we couldn't do it without some form of consent 
  > and disclosure," Bejar says.