A couple of up to date studies about Google+ are subject of an article at Marketing Charts, which do show both strong and weak parts of the performance of the Google social network.
The last sentence is symptomatic, mentioning Google+ as the Google social network. To us it is rather a matter of course, but not to all. A study interviewing the US population has shown that about 20 % are using Google+, but 20 % of those surveyed are not aware about the social network or its functions.
A dramatic growth is shown for the acceptance of Google+ among website owners, who eagerly are integrating Google+ buttons into their pages. Already in December the distribution among the US population had a reach of 3 billion page views and the landing page aka Google+ homepage had been visited by 40 million unique visitors generating 400 million visits.