For what it's worth (now THERE is a future folk song!), this is a quote from Tom Glazer in his book "A New Treasury Of Folk Songs" written in 1962.
"There are many definitions of a folk song; here is mine. I believe it is accurate and scientific in the sense that it attempts to describe an objective situation rather than subjective likes and dislikes. It is this: a folk song is almost any song which is liked by almost everybody in the country of its origin over a long period of time. This rules out: (1) songs which are or were popular for just a short time; (2) songs which may become folk songs but are not yet old enough and (3) songs which are liked by a minority of the population rather than a majority".
Why does this statement remind me of the Monty Python sketch "My name is Anne Elk and I have a theory".