I think that's one of the exciting things Fridge has done:
Provide one of the killer apps of Google Wave — things like planning a trip with friends and sharing the photos afterwards — but with e-mail integration, not in a walled garden and without all the extra cruft Wave added (threading, branching and histories).
Is the fancy Wave stuff cool for hackers? Maybe. Is it good in a product? No. I'd go far as to say Wave is to Fridge as Linux is to OS X -- Wave might be important, but not in actually using it :)
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:
Google Docs already has the most important feature from Google Wave (allow multiple people to edit the same document simultaneously). Wave's mission statement was a bit grandiose -- it should never have been about replacing email completely, the killer app is real time collaborative editing.