Jeremie Miller wrote a popular open source IM server and created a specification for the protocol which has been widely implemented in several different languages (Erlang, C, Python to name a few) for both client and server.
Many of the functions solved by XMPP (instant messaging, presence, federation) fit nicely into the distributed social network problem domain.
So that's why I have confidence that he knows what he's doing. Nits like "I don't like different XML namespaces" more or less miss the forest for the trees.
You mention that XMPP is widely implemented on the server, but only a few of the implementations are actually any good. To make matters worse, some of the best server implementations are written in fringe languages (Erlang for example).
I think that XMPP would be more successful if it had been more developer friendly.
I'm impressed by how elegant the protocol is. I hate XMPP, but telehash gives me faith.