Sol's definition of a "folkie" got me thinking of the "folk" acts that I listened to and read about in Sing Out! during the Great Folk Scare of the late fifties into the early sixties. These included Muddy Waters, Johnny Cash, Lightnin' Hopkins, Bob Dylan. . . all of which used electric instruments and made the cover of Sing Out! We called them folk musicians.
The instruments you play don't define a folkie.
Anyone who has read many copies of Sing Out!, The folks song magazine, will recall that the readers and writers of the magazine have been in a constant battle to define "Folk Music" since the fifties and perhaps earlier. I don't believe they've come to a consensus yet.