I am planning to create some dynamic content in a Facebook tab for my fan page. It should have content displayed if the visitor is not a fan, and then upon them clicking the Like button, changing the content to display hidden carrots (metaphorically). Is it possible to capture the event of the click on the Like button from inside the iframe?
On a Facebook fan page, when the user clicks the Like button, the whole page gets reloaded and Facebook sends an HTTP post to your website with a parameter called signed_request that you would need to decode and look at with server code, not javascript. The code to decode it will obviously vary depending on what language you are using, but the process is documented on Facebooks site. Once decoded, you will need to look at the page.liked value.
来源：https://stackoverflow.com/questions/6227504/how-to-capture-javascript-events-on-a-facebook-pages-iframe-tabs