Hey. Sorry for a majorly delayed response. Looks like I have two rules for this. They might be able to be optimized into one, but - I'm lazy too.

  com##IFRAME[src^="http://www.facebook.com/]

  ##IFRAME[src*="facebook.com"]

edit: Note - you need the 'Element Hiding Helper' that goes with Adblock Plus to do this. Then you can just use that to select the iframe on a site and add the rule through a nicer interface.