It's not clear how many of those people are active. With its last update, Instagram noted that 300 million used its apps daily. Most of the 600 million total are likely very active, then, but there's a chance that some of its new users only occasionally check things out.
The service doesn't directly attribute its success to specific factors, but there are a few factors likely at work. For one, Instagram's obsession with beating Snapchat is likely paying dividends. Why split your time between two services when you can create Stories or send disappearing photos in the same place you share many of your other shots? Twitter's decision to effectively kill Vine may have helped, too, by drawing in people who needed a new home for their looping videos. Throw in increased media use of Instagram and ever-improving phone camera quality and it's easy to see why Instagram would have room to grow. The question: are Snapchat-like features and live streaming enough to keep the gravy train going?