I'll reiterate what I said on the other thread: IMHO to define (and therefore classify) a folk song, you have to define the folk group it belongs to (excuse the dangling participle). This would of course take into account time as well as places as well as any other defining characteristic.

Agreed, Bert, singer songwriter stuff doesn't belong in folk unless it migrates there on it's own.

About oral transmission: do recordings count? if not, did a lot of folk music cease to exist as folk when Edison invented the phonograph? If someone is talented enough to learn a song from a recording, isn't that as valid as learning it from the singer in person? And BTW it doesn't preclude variants - see the "mistakes in listening to lyrics" thread.