Of course all songs are written by someone. It's not the writing that makes a song folk, it is how the community uses it. Silent Night is a composed song. WE even know who wrote it and when. It is a folk song because an identifiable group adopted it as their own. Happy Birthday is a composed song. It was, God help us, copyrighted, but it didnt become a folksong until folks sang it with a birthday cake chaser. Folksongs are unique. They are defined, not by their authors, but by their audiences. Folksongs are functional tools of communal practice. They are,as a rule, used in ritual. They may be employed to bolster a cause, to amuse and involve and educate (consider the A-B-C song that is sung to the tune of Twinkle Twinkle, Little Star). Woodie Guthrie didn't write folksongs but many of his songs became folksongs when they were sung by the very folks he was writing about. That's why This Land is Your Land is a folksong and The Great Philadelphia Lawyer is not.