In early 2010, Facebook made some modifications to the API which made the embedding of iFrames more restricted. Although they are no longer supported in Static FBML tabs, iFrames can still be implemented on application Canvas Pages. However, rather than an iFrame loading automatically — as it previously did on Canvas Pages — a user action — a click — is now required to load the iFrame.

In a PowerPoint that Facebook circulated in late 2009, Facebook explained the upcoming changes:

iFrames
Preserving iFrames (on a custom tab)
If you currently use an iFrame in your page and must preserve the content of one or more of your iFrames: Create a wrapper application that lives on a canvas page. This application can have iFrames since it sits on a canvas page. Have this light weight application include the image of what you want to display, with an “Activate” button. Link the “Activate” button to the canvas page wrapper application. More information here.

And here is that “wrapper application,” written in JavaScript:
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