Google+ continues to grow and the growth rate is impressive, according to Google's stats. Back in December, Google announced that "235 million [people] are active across Google (+1'ing apps in Google Play, hanging out in Gmail, connecting with friends in Search), and 135 million are active in just the stream." Now Google says that 540 million people are active across Google each month and 300 million people are active in the Google+ stream.
The number of Google+ users doubled in less than a year. Even if you use the conservative stream data and conclude that Google+ only has 300 million active users, that's still a lot. No other social network had so many users in 2 years and a half. Sure, Facebook has more than one billion active users, but the service got to 100 million users in 4 years.
Google has more than 1 billion users, so that's the upper boundary for Google+. It's unlikely that all the Google users will join Google+ and actively use it, but most of them will. Google+ is integrated with so many Google services and the YouTube integration will bring a lot more Google+ users. Google+ is now at the heart of Google's photo service, messaging service, local search and it will soon power core features of the most popular video sharing service.