Personally I prefer the Bert Lloyd definition that it is as easy to define a folk song as it is to depict the exact moment that dawn breaks and night turns into day.
When I saw Seth Lakeman at Off The Tracks festival in 2005 he played two elongated sets on the open air stage when it was bitterly cold and blowing a gale which kept most of the attendees either inside or huddled around the periphery of the courtyard. This meant that he was virtually playing to a wide open space with no apparent audience. Not once did his standards drop neither did I hear one word of complaint;the term "folk hero" might have crossed my mind on that occasion.