Find out exact segment or group of users that are converting. Locate similar people online. In fb, Reddit groups or Quora or wherever. Write articles or courses

Find out exact segment or group of users that are converting. Locate similar people online. In fb, Reddit groups or Quora or wherever. Write articles or courses to help them. Try research oriented newsletter Get audience and rest will follow. Also try to look for viral loop within your app. For eg drop box had a auto viral loop when you share files with others. Look for such opportunities. Since you are not going to spend on ad, viral loop and content are only ways to grow.