Just picked up this thread- Firstly I want to mention that in some music textbooks, the primary division (in biology terms if you will, the "Kingdoms") in the taxonomic tree of music is between "Classical" and "Folk". The orders and species branch from the tree from those two points. Clearly from this definition, folk is everything from rock and roll to country to R&B. But that's not really what we're talking about here-

Secondly, I think that definitions of style or format (as opposed to genre) are more determined by their audience than anything else. As much as we might dislike labelling music, the process serves a purpose to the listener (usually to a greater degree than the performer). Music as an industry has forced the point. What's the difference, really, between Lee Ann Rimes, Tori Amos and Janet Jackson? Their audience, certainly more than their instrumentation or vocal style or songwriting.

So my last point is, I guess, that if you're having trouble deciding whether or not you want to be known as a folksinger or a singer/songwriter, just decide whether or not you want your audience to sing along.