A folk song has to be sung by those who are outside the music business genre.
A popular song can cross over. My nominations are:
1.   This Land Is Your Land (known by every school child internationally)
2.   Maybe Blue Eyes Cryin' in the Rain (since there's Czech version.
3.   Country Roads has gotten world wide recognition

4.   Old Dan Tucker was a walk-around cakewalk from Daniel Emmett on the New York Stage, popular in its time.
5.   Dixie also by Dan Emmett from New York stage stolen by a New Orleans publishing house emerging as a phony theme for the Confederate cause.
6.   Johnny I Hardly Knew Ya' from Irish with vaudeville roots.
7.   Songs by Joe Hill such as The Preacher and the Slave.
8.   Scarborough Fair (with a bridge written by Paul Simon
9.   We Shall Overcome (from the tobacco workers union in Tennesee with roots from Rev. Tinsley called I Will Overcome.
10. Angelina Baker by Stephen Foster changed to Angeline The Baker in Appalachia.

There are many more examples and brings forth the argument as to whether folk songs are part of the aural tradition and variations or is the original song composed?
There are arguments for both sides.

Johnny B. Goode is not one because of copyright restrictions, there are no definable or recognized variants.