Singing in the shower or alone in the car etc. certainly is a mark of popularity in some sense. However, it seems that most such singers only sing a short passage, and do not intend it to be a complete musical experience.

A "song that is being sung by folks" must be sung essentially completely (though some verses may be missing) and with full deliberation. As we saw on other recent threads, some pop songs made it into that league, but the barriers are high. Pop producers want "fans", not embezzlers.