I wonder if you have a sense of how many people are into “bookmarking” mp3s of music? I remember when the system:filetype:mp3 tag was first unveiled, and it seemed like there was a big wave of tagging with it.
But now it looks like only about 10 people are tagging music with it on any given day. Of course, we should assume that something like 10x that number are reading / listening but not tagging.
But, is that really it? 100 people a day listening to music on Delicious?