I suppose that in some ways, this is just a battle of semantics, and discussions like this become tedious when they dwell on semantics. If it's just semantics, it really doesn't matter.

I think, though, that we generally share the idea that a folk song is one that has become part of our culture and has taken on a life of its own, changing in ways the author had never expected. Yes, songbook and recordings tend to create "definitive" versions of songs and to affect or even destroy the "folk process" in the life of a song - but the folk process still continues on some songs.

I suppose the purest examples we still have are songs made up by kids. Even that has changed, however. We have a great thread currently active, Jingle Bells, Batman Smells. Now, here's something you would think would be a true folk song, because certainly nonbody would want to record or publish a song like this. Trouble is, somebody did. Bart Simpson sang it on the Simpsons cartoon show, and his version suddenly and forever became the definitive version of the song.
Damn. Another good folk song down the drain.
-Joe Offer-