REM is one of those weird bands on the fringes of "folk" that seem to spring out of the "Alternative" scene (Violent Femmes are another one, but that's another discussion)
If you accept one of the definitions of folk (and let's please not get into the big "what is folk" again... this is a tentative definition used to illustrate a point) as music that gets passed down, mutates, and generally outlives the author, REM may have a shot at it. Most acoustic guitar players I know usually have at least one REM cover in their rep, and I've been to far too many parties where when "End of the World" comes on, everyone from 20-50 chimes in and tries to sing along with their own version.
Wait and see, I guess. Maybe it'll last, maybe it won't.