Well not looking into the Facebook API, it must filtering out the "/Fb:submit" information and performing a function based on this information.
However, if you do have a URL (e.g. http://www.some-website.com/index.ph...t1&info2=test2
), the information placed after the question mark will be placed within the $_GET array. In this example $_GET["info1"] = "test1" and $_GET["info2"] = "test2".