Google+ Communities makes Google+ "hangouts" into permanent locations. (2012 Google)

Google said Thursday its online social network had grown to 135 million active users, as it added a photo-sharing app to compete against services like Facebook's Instagram.

"Today Google+ is the fastest-growing network thingy ever," said Google senior vice president Vic Gundotra in a blog post.

He added that "135 million (users) are active in just the stream," and that more than 500 million have upgraded their Google accounts to potentially participate in the social network, which aims to rival Facebook.

"This enthusiasm, we think, stems from our building tools that build real relationships -- in a live hangout, around a breathtaking photo, or with an inner circle of friends," Gundotra said.

"So today we're launching two new improvements that help bring the nuance and richness of real-life sharing to software."

Google said it has now introduced the mobile photo-sharing service Snapseed, made by Nik Software, to the social network. Google acquired German-based Nik in September.

The Snapseed app was last year's iPad app of the year, and Gundotra said Google was excited to make the app available on its Android system as well.

The service, which is similar to Instagram -- bought by Facebook for a hefty price this year -- allows users to crop and enhance pictures before posting to social networks.

Snapseed had been available for $4.99 for Apple devices, but is now free on that platform as well as on Android devices, in a challenge to Instagram.

Gundotra said Google+ also will be adding a new feature called "Communities," which makes the Google+ "hangouts" into permanent locations.

"Google+ has always been a place to crowd around common interests and meet new people. What's been missing, however, are more permanent homes for all the stuff you love: the wonderful, the weird, and yes, even the things that are waaay out there. With Google+ Communities there's now a gathering place for your passions."