This list puts US at ~297m English speakers which is the largest group from one single country, that is true. But 297m / 1,537m = The US has 19.35% of English speakers globally.
You are likely also greatly underestimating current internet connectivity, older smartphones have changed things for poorer countries a lot over the past decade. For example, India has only 62.6% of people as internet users - but that's still 880m people and probably most of their 125m English speakers. Nigeria has 63.8% internet users, but that's 136m internet users. And they also have 125m English speakers, who again, are more likely to be the people who can afford an English education, and also a smartphone. And then there's Pakistan with another 100m English speakers and 70.8% internet users, etc.
Just 3 countries, (2 of which were 1 country 80 years ago) and you're close to that 300 million count already.
The list also gives US as 92.4% internet users, for what it's worth. A little less than 97% and not even in the top 20 countries by percentage, which is surprising.
The internet is less American than ever. It's just that most non-American people probably have non-English language spaces they can choose to gather in addition to the English-dominated spaces. Americans, on the other hand, are more likely to be monolingual English speakers and so they concentrate in the English-dominated spaces.
And non-Americans are all so used to people assuming American defaultism in English-dominated internet spaces because it was historically hugely expensive to get online and was overwhelmingly American English-speaking, that it's not even worth correcting when it happens the millionth time.
I've also put non-metric and US currency conversions in posts online many times. Not because I'm American or use them in daily life. It was just less annoying to convert them when writing rather than hear the inevitable multiple complaints about not understanding things in meters and dessicated jokes like "that's probably $2 in real money".
You're either overestimating the accuracy of your assumptions about your online interactions and/or seeing selection bias from your immersion in otherwise culturally isolated spaces.
For anyone else also interested, I went and had a look at the links Dessalines kindly provided.
The source on the graphs says "Sources: Daniel Cox, Survey Center on American Life; Gallup Poll Social Series; FT analysis of General Social Surveys of Korea, Germany & US and the British Election Study. US data is respondent’s stated ideology. Other countries show support for liberal and conservative parties All figures are adjusted for time trend in the overall population." Where FT is financial times.
It's not clear how the words "liberal" and "conservative" were chosen, whether they're intended to mean "socially progressive" and "socially traditional" or have other connotations bound with the political parties too, and whether the original data chose those descriptions or if they're FT's inference as being "close enough" for an American audience.
Unfortunately the FT data site is refusing to let me look at them without "legitimate interest" advertising cookies so I can't tell you much more or if there's any detail on methodology.