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This isn't the article I was looking for, but it presents the main problems: http://www.usatoday.com/tech/news/story/2011-11-15/facebook-privacy-tracking-data/5

• hellotoast 15y ago
  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.