Dear Peter,
This is a very valid question to be asking yourself. The great thing about digital marketing is that a lot of the work can be done in-house and the results can be easily and immediately quantified. The channels you choose to use really depend on your target audience. That said, in our region of the world a large number of consumers still read newspapers and watch TV therefore you should not ignore these mediums altogether.
Generally, the most effective marketing plans utilise a variety of channels that together ensure the highest exposure to your target audience. With apps, one could safely assume that your customers would already be relatively tech savvy and therefore be exposed to do social media channels.
With more traditional avenues and over the long term, public relations (PR) is a medium that can be tailored to target your specific audience. That being said, in the start-up phase of a business, when you may not have a big enough marketing budget that allows you to use multiple mediums, it is of course good to use the most effective and measurable methods. Once you pick up good momentum, you should definitely start looking at other channels.
Look forward to learning more about the apps you are producing.
Best, Badr