(Important Update: Facebook no longer supports FBML. Please refer to the articles on Facebook iFrame.) One of the most popular FBML tags is fb:visible-to-connection. A favorite of marketers, this FBML tag allows a Facebook page to show different content to fans and non-fans. When a non-fan clicks the Like button – viola! – the non-fan content disappears and the fans-only content replaces it. As a method of motivating a visitor to become a fan of your page, this can be very effective.
This FBML tag is often—and erroneously—referred to as a “hack”; however, it was created by Facebook to do exactly what it does: ”to display the content inside the tag on a user’s or a Facebook page’s profile only if the viewer is a friend of that user or is a fan of that Facebook page.”