Cause XMPP is pretty broken
Maybe try inspecting the site php [blah]
Although it only takes live messages, so it can't load up the chat and, more importantly, if your internet blips out for a bit, you can miss messages from the XMPP while the website obviously maintains them.
You can't just inspect a website's php code...
What? XMPP is a highly standardized, widely used and extensible instant messaging framework.
This is an issue with Facebook, not XMPP.