Just a few weeks after hitting 30 million registered users (on April 18), the app reached the 40 million mark on June 6. Growth seems to slow down (NHN Japan reported the 20 millionth user on March 5), and there is a lot of marketing power behind the success now, but still, these are amazing numbers.
The first version of LINE was released in the Japanese App Store on June 23, 2011. In other words, the app added 40 million registered users in under year.
LINE has over 18 million registered users in Japan alone. This chart shows how quickly the app grew over time: