Google, of course, disputes the point. They consistently report that Google+ is booming, with numbers going through the roof. The numbers they choose to report, however, are suspiciously vague.
The company has been asked repeatedly for monthly active users, and it's repeatedly denied such requests, essentially calling them irrelevant. The closest we've seen of active usership was when the company explained how many Google+ users were engaging with Google Plus-enhanced or -related products. The problem is that Google Plus-enhanced products include YouTube and Google.com, meaning if you are engaging with basically any Google property (there are 120 Google+ integrations thus far) while signed up with Google+, Google is basically counting this as engagement with Google+, which is incredibly misleading.Google is, essentially, playing the same game that Amazon plays with Kindle sales numbers. Amazon constantly brags about how great the Kindle is doing, and yet to date has refused to disclose how many, precisely, have been sold. They do so in the knowledge that some lazy reporters will pass on the headline without reflection: "Kindle sales going up, up, up!" But those who are paying attention ask: if the product is really doing well, wouldn't you want to be specific about how well it's doing?
Google is undeniably playing games with its numbers for Google+. Counting engagement with search, Gmail, and YouTube as if that were intentional engagement with a social network is disingenuous, and that begs the question: what motivates that behavior? Success tells its own story; if the numbers are really good, you'll share those really good numbers with anyone who will listen. When you brag about your success while playing games with the numbers, it makes you look like a liar.