It's hardly controversial to point out that Google Plus hasn't set the world on fire. It's interesting to see what's doing better, though. This Google Trends chart shows global search volume for 'Twitter' (blue), 'Tumblr' (red), 'Google plus' (green) and 'Google+' (yellow).
People who've actually worked out new and interesting social forms are doing rather better than a top-down attempt to 'fix' perceived gaps in Facebook.
App Annie gives another perspective on this: where does the iOS Google+ app sit in the rankings?
The only country where Google+ is top of the social networking category is Albania.
Of course, 'Google Plus' is arguably a catch-all name for a project to get all Google users to be logged into a unified system and, as importantly, to stay logged in when they leave Google: that way any page you visit with a 'share on Google Plus' button will be linked to your ID and allow Google to improve targeting. To that extent, getting people to go to the Google Plus homepage isn't the main objective.
The ultimate expression of that, of course, is Android, which, by default, tracks all sorts of things you do. Google Maps for iOS doesn't store a search history unless you log-in: a clear case of putting corporate objectives ahead of user experience. But what happens when you need to log into Google to, say, watch Youtube videos on mobile? Or to see reviews on Google Maps? There are all sorts of levers that Google can pull, with very varying results, much as Microsoft could two decades ago.