I doubt your idea in importance of big players in the internet.
At one hand, it's much easier for a website learning idea and technology from a successful website and errode its market share than in other industries. And as information provider, it's much harder for a website having high customer loyaty, because it's easy to find information elsewhere. Hence, it's much harder for a player remain its importance.
At another hand, in this era of web 2.0 and blogging, what's important is the the small website such as forum and personal blog. Many people no longer go to the top website, but instead they go to some specialized website. It's a process of decentralization and customalization. You can't just look at the top 500.
Take China for example, the number of people who surf internet frequently increased about 25%, and the average time increase 1 hour per week in 2006. Taking into the consideration of the declince of important website, there is a clear trend of dencentralization to small website.
I think if you look at the traffic distribution trend among the top 5000 or 50000, you may get some more interesting result.