Population of U.S. ~340 million.

Population of Canada, UK and Australia combined: ~138 million.

Chances are, native English speakers on the internet are American.

I don't assume people who speak other languages on the internet are American, but English speakers probably are. Unless they're in country specific communities, like Canada, or UK. I do check the community before I butt in with my American take in country communities, usually, I don't comment at all. But, I do see my fellow Americans making fools of themselves in those places sometimes.