I think one of the most important things to know about marketing anything, is that it's not about activities, it's about results...and knowing what activities bring the results, and at what cost.
A few critical marketing metrics would be important to learn about, like cost per customer (how much can you afford to pay for a customer....it varies based on your price/profit and it's different for each company), or LTV, or MRR, etc.
For that reason, it's not simple to say what someone should do to market their app to their customers. Something that works for one app, may not work for another.
Sometimes free things work, sometimes they don't. Sometimes free things that don't work can cost (time, resources, opportunity cost).
That's why tracking and testing in marketing is so important...so you can continuously learn and evolve based on results. If you don't track/test, then you may be engaging in fruitless activities, or in the case of social, sometimes chasing "vanity metrics" like followers, likes etc....which may not translate to revenue.
If you're interested in learning more about marketing, the Growthhackers community is a decent place to start: growthhackers.com/posts.