It's generally what's people are willing to spend on exposure, in lets say commercials, calculated for one year. It's an estimate on what you would earn from adds in one year.

Introducing ads might make the users leave though, so be careful. There's also a risk Facebook would have something to say about your monetizing on their product.

So my suggestion is to implement that feature ppl want, let them know about the awesome dude behind it, and that you care about their user experience. Then leverage the goodwill it creates to get free exposure and users to your other product(s), that are monetized. Basically you can use the app to build a loyal user base.

An example is the dude who made Flappy Bird. He also got lots of downloads and exposure for his other games just by having his name on it.