I would say that the us accounts for less than half of English speaking netizens.
UK, Australia, new zealand Canada already add up to a sizeable chunk of the US's population. Then add in 1% of India, or China (I assume some proportion has access outside the great firewall) and you've got a population bigger than the US.
If you add up the numbers of the countries you mentioned (together with 1% of India or China), you'll realize that it's smaller than the US population...
US: 331.9 million
UK: 67.33 million
AU: 25.69 million
CA: 38.25 million
NZ: 5.123 million
IN: 1.408 billion * 0.01 = 14m
Point is some very small percentage of the rest of the world is going to dwarf the US. Yes the US will be the single biggest demographic but less than 50% of the total.
I haven't been able to find many solid stats, but eg English language Wikipedia, the US has 22k active editors out of 59k total.
Disclaimer: Not English. Not American.