I think you need to distinguish between at least two ways of learning here. You seem to be more concerned with those songs picked up by familiarity subconsciously as opposed to those picked up by deliberate learning/practice with a view to perform. Towards the end of the last thread Brian described how youngsters nowadays are easily picking songs up by repeatedly playing them on modern gadgetry. From what he says this does not seem to be as selective as deliberate learning as they are picking up whole albums by heart, not just say the best song on the album.

Certainly familiarty/constant exposure seems to be a major factor in what we pick up. Almost everybody can sing over the TV jingles due to constant exposure and the clever use of existing tunes, sometimes even folk tunes. (We all love Clover..quick ad).

Once we pick up a song it's hard to get rid of it. It stays in our unconscious mind. Good example is as Bernard says, the earworm, although that is usually just a tune. For almost a year I went around humming a Fado tune that had been popular in the 50s. I even set words to it before I eventually tracked it down.

Much of my repertoire as a singer is songs I have picked up in the clubs through constant exposure, Fathom the Bowl, Adieu Sweet Lovely Nancy, Collier Brigg, numerous shanties, rugby songs; but by no means all. Of course tunes are mostly picked up orally anyway as, like me, most people don't read music.