Year-old Google+ has passed a significant milestone and Google announced 400 million users, with 100 million of them "active" each month, according to CNET.

"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," said Vic Gundotra, senior vice president of engineering at Google, in a post. "While Google+ is all about creating a better experience across Google, it's also a destination."

But questions remain over how active those 100 million users really are, CNET says. Google counts anyone who has signed up for Google+ as a "user," and anyone signed in to Google who interacts with a product "related" to Google+ - YouTube, Google Play, and the Google search engine, to name three - as an "active user."