I was thinking the exact same thing. It could very well be a good idea that is ahead of it's time. We are still wedded tightly to traditional email, but something like Wave could very well take over in the next say, decade or so.
Indeed, Facebook could provide a collaboration model for the future. I don't think the collaboration platform will be Facebook itself (not their focus) but perhaps a startup will come along, look at how people communicate on Facebook, and apply that to business. Wait - there are guys doing that now (I love how framing something by typing it out fleshes out your thought process!) I saw a good presentation on HN on Enterprise 2.0 recently, I'll try and find the link.
EDIT: Jive are one example, not the link I had in mind though: