I know, let's all discuss this on Google Wave! After all, Wave has massive potential for business users, http://mashable.com/2009/12/18/google-wave-business/. With 19 Educational uses, http://www.soyouwanttoteach.com/the-power-of-potential-19-educational-uses-for-google-wave/.
Unfortunately, I can't find the uptake numbers for Wave. Of course, just because one product flops doesn't mean the next must too. It's just that one of the reasons Wave probably failed was that it didn't offer people anything they weren't already getting somewhere else and they were too entrenched to change. People who needed real time collaboration already had mature products available to them, Elluminate, Contribute (or whatever it was in 2009), Live Meeting, or even GoTo Meeting. For people who didn't need the collaboration, Wave was an answer to a question no one asked. Even in 2009, Facebook was "good enough" for people.
So what about Google+? Does the minor difference in features warrant changing off facebook? Probably not. Does it offer anything outstandingly new or innovative? Probably not. Are people even more entrenched in their facebook lives now than in 2009? Probably. Add to that the real name policy and the inability to work with non-european names and there's even less reason to move.
Way back at the dawn of time, when Google was just opening its eyes, it was competing with some really big search engines. Remember how big Yahoo used to be? Or AskJeeves? Google didn't bring anything new to the table, but they were able to compete by being better. And switching search engines is much easier than switching social networks. When they competed on the email front, they did it by giving people a ton of storage. When Hotmail was offering storage in the megabytes, Google was offering it in the gigabytes and even Hotmail had to play catchup. People hate to delete emails, and Google let them keep everything for ever and never clean.
The other two big products, maps/earth and image search, weren't really competing against an entrenched alternative. There was mapquest, but even it was new.
So my armchair quarterback position is that G+ will peak very soon then slowly decline until in another year or two we'll be talking about G+'s failure. Which will be right around the time Google announces G++.