The idea is simple: you track favors that your friends are doing for you.
Do you see this as complete nonsense, or do you find it a viable usecase many people have?
Note: I have a lot of code already written, but before I proceed I need check if people are positive about it.
That said, I think you should still finish the app. Startups pivot a LOT, and you might stumble across an awesome idea once you start looking at feedback and trying to figure out what people really use the app for.
That all said I don't do facebook and this is something that would bolt on there easier than most. Maybe look at g+ as a interface platform.
Good luck, and nomatter what people say, see if it floats would be the way as you have already done all the work etc.
That being said, some people might still find it interesting.
I might actually use the service in reverse to, but more in a business context. So and so owes me 1, because I did this for them. A sales rep I worked with once gave me 15% on a future order (I could call in that favor whenever), because of some screw ups, so I would definitely record that.