I'd be really surprised if usage was that high. I'd believe there being maybe a few thousand using it regularly.
It wouldnt be a stretch for active users to run into the high tens of thousands. Unfortunately, thats tiny compared to the usage of Google's high-profile projects, so it gets killed.
Wave's problem wasn't that it's not useful, it's that nobody knew about it (of people I know, it had 0 adoption outside of computer geeks), and it couldn't integrate with desktop applications. A lot of us are stubbornly hanging on to Mail.app and Entourage because they integrate with our workflow better than web based solutions.
If google had come out with a a desktop Wave client that worked with OSX's address book, photo libraries, etc, it would have had a lot more draw. That sort of integration is less expected on Windows, but I'm sure they could have come up with some compelling features that the website didn't offer.
The Google Wave example is a telling one - Wave wasn't the success Google wanted, so they scratched it. Nevermind that there were great uses for Google Wave, and there were probably hundreds of thousands of users on it, at least, who really liked it. In Google's eyes, Wave didn't succeed in becoming the huge hit they wanted, so it was a failure.
A small startup would have considered it a huge deal and pivoted on Wave's good uses until they turned it into a much larger project, IMO.