Again, product versus framework. XMPP as a communications framework has decent adoption, a number of open source servers and clients libraries in different languages. Jabber has limited adoption because its competing in a network-effect field. GoogleTalk is probably the most popular use of XMPP.
But, the meat of Wave isn't XMPP, its the combination of Operational Transforms on a federated XMPP layer with a web interface. Each of these features is equally important, so don't overlook them. OT makes collaboration easy, the web interface is the easiest user experience, and XMPP lets you scale the backend.