Apart from when talking about performers of traditional material, I don't think it's possible nowadays to wholly define who is and isn't a folksinger. I'm not sure it really matters. There does seem to be a massively arbitrary element to it.

I'm with Howard on the "linguistic slippage theory". I also think there's a point to be made about context. For example, George Papavgeris and James Yorkston are both contemporary singer songwriters. George is considered a folk singer, because he often plies his trade at folk clubs and festivals to a folk audience. James isn't because he usually doesn't. However, to the general public, both would be considered folksingers and to the 1954ers neither would. Context and perspective, then.

Blooming good songwriters, nonetheless.