Do I build a mockup page and advertise it on the right side of the Facebook page, along with other ads? Do I make a survey of potential customers to see if they're willing to pay for the app (SurveyMonkey, or smtg else?)? Should I build at least MVP to give potential customers a taste test? Thanks.
You're on the right track. I'm glad to see you getting the steps in the right order.
WHO are potential users of your product?
WHAT problem(s) will your app potentially be solving for them?
HOW much do these potential users value the solution of these problems?
Find your user group of people who would pay for a solution to a problem your app will solve. Get their input. I would do this through a couple stages...the first involving live talking, and the second usage of a basic version of the app once you and they have figured out what its purpose is.