Is Facebook Markup Language (FBML) HTML, XML or some homemade demon spawn of the two?

Is this some new stage of XML's ubiquity, where XML is such a given it does not even need to be stated, let alone explained. As near as I can work out, FBML is designed to look like XML but not necessarily be well-formed. But I really don't know...Does any reader have any pointers to better information?

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