The Big question is how do you get people to use the app -

There is a tonne of ways really, but it really depends on what you want - more users is very broad

Who is your target market

age, gender, demographics

--- once you have this ---

where do you find these people - communities? social networks?

what apps are they already using?

what simliar apps to yours have the largest active users?
How long have they been around?
how do they market their app and to who?

what is your engagement rate of the apps users?

How long do they use it, when do they close it, do they uninstall it - if so why?

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once you have all these questions you can at least start te basic marketing -

be careful paying for marketing most are complete scams - dont try and cheat active users - imagine it like this, someone sends me your app - if i dont meet the target market above then you app has 0 chance with me - and this is the case for most - aim for targeted marketing - Your app is your business - your active users are your customers - market your business to the right customers, treat your customers right and over time you cannot fail.