Which of the many popular songs do you remember from last year?
Not too many right?
Now think about the songs you do remember, and see what comes to mind.
This is just a hypothesis but I'm sure it can be proved if tested enough times. According to me, the only songs we remember are those which can be related to a memory of something or someone. Now, that 'something' could just be the music video or a picture of the artist. Strangely enough though, some of these songs are related to something in our lives that is important to us or used to be of some importance to us, the songs could also be related to some special person in our lives.
The cause for this, I think, is the brain's way of categorizing the important bits of information from the less relevant data. This doesn't necessarily mean that the song is expelled from our memory, it means that those bits of information get stored into the deeper parts of our subconscious mind, which we can't access easily. It can be noticed though, that after hearing a few lines from a song you heard long ago, makes that song pop up in your head almost immediately. That is what might be called as the brain's 'recovery' of data from the 'archives' of the memory.
For example, I remember Michael Jackson's song, Billie Jean, because of the fact that I danced on it during a school organised function and also because it is one of my favourite songs of MJ, and it always reminds me of him.
Also, it doesn't necessarily need to be a full song, even a simple tune could be embedded in your memory if it is related to something relevant enough.
For obvious reasons, the memory of the more recent songs can't be expected to follow the same pattern. Since the recent songs are talked about amongst friends, family,etc. and they also pop up on cable television and of course, the internet, these songs never really leave our immediate memory until some time has passed.
Speaking of songs, here is a great one!
(Note for the dumb: I was being sarcastic about the video)