If you want to understand what makes a successful Social Media App or a Messaging App, Twitter should be on your list to study. Their trajectory was not as stellar as Facebook but that gives us a great opportunity to learn the traps to avoid. But with about 300m monetisable daily active users, they are still one of the most successful Apps around.
The key ingredient to getting to such a user base is not the App as such. It is the business model behind the App. I have the feeling that many developers and even entrepreneurs are focussing on individual / cool features of an App. I recommend not to waste years “developing” into the blue. First and foremost, understand the business model that will power your App. You can do this in parallel to developing your App.