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I've done some XMPP development so when I read Facebook was making a Jabber chat client I was really curious how they would make it work. While core XMPP is straightforward, a number of protocol extensions like discovery, forms, chat states, pubsub, multi user chat, and privacy lists really up the implementation complexity. Some real engineering challenges were involved to make this puppy scale and perform. It's not clear what extensions they've implemented, but a blog entry by Facebook's Eugene Letuchy hits some of the architectural challenges they faced and how they overcame them.

Large scale distributed instant messaging, presence based protocol are a real challenge. With big players adopting the standard, the XMPP (eXtensible Messaging and Presence Protocol) community is facing the need to validate protocol and implementations to even larger scale.