For myself, I don't have too many outstanding favors at any one time. I tend to know who has helped me out. Even had I forgot, I'm sure just talking to the person (or at least when they redeem their favor), I'd remember.

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.