Google+ has 400 million members, a quarter of whom are active each month
 

Despite a fashionably late entrance to the social networking party 12 months ago, Google+ is already able to claim 400 million users and 100 million monthly active users across both the website and the mobile apps. That compares to just 250 million total users back in June, suggesting the service is growing faster than ever. Revealing the figures as part of his announcement of Google's latest corporate acquisition, a victorious Vic Gundotra said that his team "couldn't have imagined that so many people would join" within a year of G+ going public.

http://www.engadget.com/2012/09/18/g...llion-members/