I definitely agree with Don that the modern 'self-written' song not being a folk song....unless it's adapted by 'the folk'.

I also think that Don's analogy of 'antiques' is an interesting one. But with 'folk' there are always exceptions....I don't know if there are with antiques. Don't they have to meet all the criteria to be antiques?

And I've heard Johnny B. Goode played by many a bar/jam band....and most of the time it's awful.   Never know how they'll do it.

Re the 100 yr. old criteria: There are a lot of commercial songs that were influenced by traditional sources....the songs aren't particularly old, but the sources or 'fragments' they took it from are.

Blandiver's criteria is also interesting. Folk music is "music played by folkies". It could get pretty circular. If a folkie brought his or her computerized equipment that s/he just bought, and started demonstrating it at a 'folk' open stage.....would it be folk music?