Aaaahh, found it!!!

Facebook documentation: (
Creates a JavaScript submission mechanism for a form, which makes image or text links act as Submit buttons. Markup contained by this tag is surrounded with an <a> tag that includes a submit onclick action.

Basically, anything contained within the <fbsubmit> .... </fb:submit> tags acts as a submit button. All of the tags that Facebook uses can be found at By the looks of things they have their own set of HTML markup tags. This is to ensure that everyone that develops applications to work with Facebook can integrate successfully.