Pidgin also supported XMPP: it was a multi-protocol client. It was just a client, not a messaging system of its own. They'd abstracted the send/receive from the actual messages and built a bunch of backends that could handle pretty much any messaging protocol at the time.

ICQ was released 1996; Pidgin (as GAIM) was November 1998; Jabber was released in 1999. Those were good times.