How facebook make the call to my callback function when i subscribe to edge.create event

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C#

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How facebook make the call to my callback function when i subscribe to edge.create event ?

I missing here something ,
when i put a like button in my page facebook create an iframe and inside the iframe
they put the like button when i click on the like button in case i registered to
edge.create event , facebook call to my callback function that live in my page ,
and obviously my page is in another domain from the facebook iframe so
how they call to my call back function

hope someone can clear me the subject

Thanks in advance