I reached this milestone by listening to my early users & tweaking the Android application to add features users wanted. This spurred more users to download the app. I also worked on localization thanks to great users. The Android is translated into 5 languages so far.
I learned how to achieve this for future projects when I can give my full focus to a new mobile project later. Growth is a little slower now but I'm working now on the iOS app more for the time being until I have more bandwidth to focus on the Android version.