I suggest adding virtual currency to this game.
I guess most income are related to advertising and other incomes are from supporters that are not expose to ads.
Virtual currency features might be premium features that doesn't provide an unfair advantage in the game for supporter and non supporter users.
Virtual currency features might be any type of silly visual addition, such as a funky hair style to a specific player, an earning or any other type of funny/cool addition.
Another feature might be having a team mascotte.
Users will be able to gain BB coins or something like that by Paypal or other type of online payment methods or by successfully referring a user to the game. Maybe some kind of a refer a friend challenge (i.e - 5 refers will gain user with x BB coins)
It might work as users are really engage to this game - hopefully it will help raising user value and enable BB to buy more users and develop the game.
"Did you miss me??? - "With every bullet so far..." Al Bundy