How does the Mainstream-Meter work?
The Mainstream-O-Meter calculates your mainstreamness by comparing the listener count of your favorite bands to the average listener count of the five bands who have the most listeners among Last.fm-users.
Of course, the results may be different from what you would've thought of as mainstream. Some bands which have a lot of listeners in real life don't have a large fanbase on Last.fm since people who use this service tend to be younger and more experienced with computers than the average person. Because of the restricted number of users, some genres like classical music and countries like India or the People's Republic of Chine are really underrepresented on Last.fm.
Since some users listen to all of their top artists equally while others have one top artist they listen to 99 percent of the time, the Meter also takes into account how much you listen to these bands in comparison. The artist you listen to most always gets a weight of 100%, while every other artist's play count in your profile is then compared to this.
So, if you would have played 100 tracks by Artist A (which is 75% mainstream) and 50 tracks by Artist B (which is 10% mainstream) this would come out as 
.