I like your definition, especially the part 'sung or played socially'. The only problem I have with that parody as being folk is that I don't really know if anyone is singing it anymore.
I personally think that a song has to be able to survive on its own merit. Which kind of means that if anyone is singing it or listening to it and it's no longer 'in the charts' it's probably folk music.
I also agree that 'virtually all perfomed folk music' probably isn't. Most of what I am hearing in Philadelphia is personal one time stuff.