Every time you visit a web page that displays the Like button Facebook logs that data in your account. It doesn’t put anything on your wall but it knows where you’ve been.
Source? I've never heard that before. My understanding of the above is that Facebook will store, on their servers, all of the pages I visit that display a Like button.
Edit: The previously linked to article in the essay supports the claim: http://news.cnet.com/8301-13578_3-20006532-38.html I was unaware that Facebook was able to retain information even if users didn't actually press the Like button.