Google said 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 on Thursday. 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.

Google said it has now introduced the mobile photo-sharing service Snapseed, made by Nik Software, to the social network. 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 allows users to crop and enhance pictures before posting to social networks.