don't see that Neil Young has got much to do with folk but the Stones undoubtedly do.they started off playing folk material exclusively, then progressed to song writing, and their first effort (I think) was "This could be the last time" was a reworking of a spiritual. They were the first to master the new technology of using big PA systems to make big audiences part of the act in a real way. And they developed a whole set of tricks which spread and were used by huge numbers of people creating all sorts of new stuff. Smack in the middle of the folk process I would have thought. Far more so than the average acoustic guitar/sonngwriter who gets classified under "folk" in record shops.