Facebook occasionally pairs advertisements with relevant social actions from a user’s friends to create Facebook Ads. Facebook Ads make advertisements more interesting and more tailored to you and your friends. These respect all privacy rules.
To turn this off, go to Settings -> Privacy -> News Feed and Wall -> Facebook Ads -> Appearance in Facebook Ads and select “no one.”
Note that this privacy page doesn’t appear in Firefox 3.5 if you use AdBlock Plus extension. Even selecting “disable on this page only” in AdBlock Plus and refreshing the page won’t make the controls appear. I had to completely disable AdBlock Plus and refresh to the page in order to see them. Alternatively, you could use another browser (e.g. Safari, Chrome).