Wave was cool when it came out, but the initial reaction (hype) seems to have died off. I used it when it came out, but I haven't loggen in for months, and none of my friends have. Either that mode of communication isn't that attractive and/or Google introduced it poorly (Google-only, crappy web UI).
You know, I think they might have been thinking along the lines of a "distributed Facebook"-type thing when this came out. Anyone looking to replace Facebook should probably learn the lessons of Wave's failure before embarking on their own project.
Wave is probably too complicated for the average Netizen. I've personally only found it useful as a sort of real-time chat. Haven't checked my inbox in months at this point.