My opinion, and I think I got this from Patrick indirectly if not directly, is that you need to be selling something to people that will cause them to receive more money than your product costs. So you cannot target language learners (especially the relative immaturity of Reddit) because it's a hobby, but you can target businesses and professionals because they routinely spend money on tools and services. Sadly that's all I know about it, so I couldn't tell you who that guy should have targeted with the app. Doctors? Lawyers?