An exchange of promotion with another app - you promote their app to your users, and vice versa - is a cheap, mutually beneficial way to reach a new and relevant audience for your app. Small or large, your user base is valuable, so reach out to complementary apps to build cross-promotion partnerships. For example, if you have a job search app, a money management app’s audience could be highly useful for your user growth, and your audience of job seekers likely need money management advice as well.