Dunno, Nirvana we're pretty big...
Either way, I like Mr Dave's approach on the matter... In ye olde days, everyone was subject to the exact same fads/ideas by mass media. If there was something gaining momentum, it would dominate radio/TV/news, and it would have an impact purely because there are so many people focused on it at once. With the internet, more people are experiencing something before the majority do, or there are more avenues for people to experience music rather than just the radio or TV. They can pick what they listen to more than in the past decades, and more often than not, its not whats being rammed down your throat on the radio.
I am sure Mr Dave can explain the idea a little more concisely
