Google said today that 100 million people are using Google+ each month, a significant milestone for the year-old social network. The company said it now has 400 million total users.

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

In a subsequent comment about his Google+ post, Gundotra clarified that those 100 million people are visiting plus.google.com on the desktop, the mobile Web, or the Google+ app. Initially it was unclear how Google was counting "active users."

It turns out that Google uses two main metrics to gauge Google+ usage. One is use of Google+ across what the company calls "the social spine" of Google. That includes things like +1's on apps inside Google Play, or use of YouTube by signed-in users. At its Google I/O developers conference in June, the company said 150 million people a month were using Google+ in that way.

Today's numbers reflect the other metric: use of Google+ as a destination. This marks the first time Google has released any information about monthly Google+ visits.

"The numbers at I/O were Google+ users all across Google," Gundotra wrote in response to a question from a Google+ user. "So Gmail users who used circles, or Android users who plus-one'd an app etc. were included. Today's number of 100m is just on our plus.google.com destination site and the mobile app. Pretty amazing."

In April, CNET's Rafe Needleman asked Gundotra how many people were using Google+ features. Gundotra told him he was "looking at it wrong."

"You have to understand what Google+ is," Gundotra said. "It's really the unification of all of Google's services, with a common social layer."

But assuming the 100 million user figure is active, a growing number of people are visiting Google+ on a regular basis. As for how long they spend there, or what they do there -- that will have to wait for another day.

Update, 12:20 p.m. This post has been revised to reflect Gundotra's comments about how Google calculates its active-user numbers. It was not initially clear that the company was including visits to its Google+ properties separate from usage of Google+ features on other Google sites.