This is just an adaptation of the original Facebox plugin to add iFrame support.
The adaptation is based on this post by ShortcutTO701
You can use the plugin as always, however if you want to display "iframed" content you should add a "rev" attribute to the link as follows:
<a href="/contact" rel="[facebox]" rev="iframe|350">Contact us</a>
The "rev" attribute has two options splitted up by a pipe (|). In the above example, the "350" is the "height" of the iFrame.