Perhaps you haven´t even thought about it. What is my mainstreamness when it comes to listening music? Some people who only listen narrow area of certain style of music may achieve a high mainstream percentage. Perhaps people who listen to Linking Park, Nelly Furtado or Madonna or perhaps other mainstream crap. Popular music is not perhaps the best when it comes to listening pleasure. It all is so sexy and loud but the music itself is all sounding the same.
It all comes to what you may have listened in last.fm or on your mp3 player in your pc. I'm not sure if there´s some spyware hidden in the last.fm because everytime you listen something with winamp your music is shown at last.fm . That could also described as Taste-O-Meter ?
Let´s make a little test. My score with current listens is 4.86%
After listening Maroon 5 and Linking Park the score is still the same. The new value should be calculated some moment later. Database cannot possibly update latest songs instantly.
Try it yourself: http://mainstream.vincentahrend.com