I got the advice from a friend to use Facebook ads to get more installs, but it seems expensive. It seems that the price for each install depends on how effective the app ad banner is. How much have the CPI been when you have used Facebook ads? I found this info here:

"Advertisers frequently ask how much they should expect to pay per mobile app install ad on Facebook and Instagram. The answer is simple: anywhere from $0.25 to $100 CPI."