The United States is the world's third most populous country, and home to a number of technology businesses–one of the main draws of the site. I don't know if anyone has officially disclosed Hacker News's traffic data, but a lot of people have done analysis of what kind of traffic they get from being on the front page and it's invariably dominated by the United States: https://nicklafferty.com/blog/what-happens-when-you-re-on-th...
1) Even if you combine the top 3, they would be quite smaller than the rest combined.
2) The actual markets is what you should look at, since you are talking about business, trade, etc. The EU is bigger than the US.
3) You wrote "most websites", not just HN. That is just incredibly wrong. Even if you had written "most English-speaking websites", you would still be wrong for the most popular sites.
