I had exactly the same experience. I was at university, and around 20% of students on my course had access to Wave, which functionally meant 0% of students could use it.
“An app to collaborate on, but nobody to collaborate with” has to be the most economically destructive product rollout I’ve ever seen.
GMail was still fresh at the time, and it rolled out in a similar manner, being invite-only at first. I think they didn't think about it very much, and just did the same thing.
But email was already interoperable. GMail offered a nice interface, lots of storage, and a good spam filter, but otherwise it was just email. You didn't need to have friends with it to benefit from it.
Having used Wave, it was very taxing on low-end computers, so I never ended up using the fancier features - we used it for a group live-watch of LOST every week with several other friends.
“An app to collaborate on, but nobody to collaborate with” has to be the most economically destructive product rollout I’ve ever seen.