>>We haven't done any serious research or marketing for it.
It could be a personal taste question, but why not any marketing? With enough traction to be paying rent based solely off word-of-mouth and organic, you certainly seem to be creating value for people. There are easy/free/cheap ways to market apps, definitely at least some amount of money being left on the table.
Congrats either way, good stuff :)
We've done some experimenting with marketing, but couldn't figure out any strategy that could convert a bride or groom for $3.50 (the amount we net after Apple's cut). We tried and abandoned Google ads, Facebook ads, and advertising on wedding blogs. Virtually all app-marketing services focus on free apps, and most of them are grey hat in some way.
For a general-purpose app (like our upcoming Party Monster, a more general music app) you don't need to be targeted, but for a wedding app you need to target engaged people, which is expensive!