Google Plus has 540 million active users

"At a Google+ event in San Francisco, Google’s VP of Engineering Vic Gundotra took the stage to discuss the promise of the company’s social network. But before it began to talk about its future, Gundotra decided to share more about where it’s going now.
Now two years old, the social network has seen a new version published nearly every day — nearly 20 launches in the last 4 months such as sync’d notifications and translations.
So how is the social network doing? There are more than 540 million active users worldwide with 300 million active users in the stream alone. There are 1.5 billion photos being uploaded to Google+ too each week."
Source:  The Next Web, 29th October 2013

Note - There is some dispute over what an 'active user' is - e.g. you're counted if you 'like' a YouTube video or '+1' a page
How the 300 million people who have posted, shared, or commented in the stream in the last 30 days is very impressive
Update - A good analysis of the definitions here on WSJ