Larry Saidman: I definitely agree with Don that the modern 'self-written' song not being a folk song....unless it's adapted by 'the folk'.
It is adapted by 'the folk'. The modern 'self-written' song is a culmination of a cultural journey. People "write" songs using popular chord progressions (which are changing all the time) and melodic ideas which are popular and people identify with. If that isn't the music of the folk then I can't imagine what is. :)
Note too the word "write". This is not generally what is happening in that genre. People compose these songs in a very non-academic way - far away from paper. The very process is a popular tradition. I think it speaks of a time and a culture. Whether a large number of people listen to, or take up, the song is irrelevant to my way of thinking. It is the culture and the process which makes it "folk".