A report on 89n.com seems to indicate that Google+ is losing momentum.
From the graphs in their post, it actually appears that Google+ is hemorrhaging interest. While Google+ has been gaining users, those users are posting much less.
If Google+ wants to be a go-to networking service, it needs to reach a critical mass like Facebook, and to a lesser extent, LinkedIn have. That’s why these services are so entrenched – users go to these services because so many of their friends and colleagues are on them.
Here is a graph:
Head to their website to see the rest.
EDIT: Please note that this was not a survey of all users, but ones that used ManageFlitter’s service.