The search engine firm's social network has attracted 400m sign-ups since it went live last year, but only one quarter of that figure apparently make use of the platform.
"This week we hit an important milestone - over 400,000,000 people have upgraded to Google+," said Google's Vic Gundotra in a blog post. "It was only a year ago that we opened public sign-up, and we couldn't have imagined that so many people would join in just 12 months.
"While Google+ is all about creating a better experience across Google, it's also a destination. And here too, I'm happy to report that we have just crossed 100,000,000 monthly active users on Google+."
Earlier this year, Google began ushering Gmail users into Google+ by automatically setting up a profile for new users. Around 75m are believed to have signed up to the company's email service during the first six months of 2012.
Google recently completed the acquisition of Instagram rival Snapseed as part of its buyout of Nik software. The photo service is expected to be incorporated into Google+.