To be fair, the US has the largest number of English-speakers of any country in the world. As a first language, it has five times as many native English speakers as second place (the UK). It also has one of the highest Internet penetration rates in the world, meaning most of those English-speakers are also Internet users.
The US is a single country that is three-quarters the population of the entire European Union, and nearly all of its inhabitants speak English and use the Internet. So yes, if you pick a random user on an English social media page, odds are very good that person is an American. If you were to guess any random English-speaking Internet user's nationality, "American" is the best possible guess. But go on a Spanish language forum or a French language forum and nobody will assume you're American.
Consequently, Americans generate the majority or large plurality of English-language Internet content.
Edit: Please stop replying with "English is a lingua franca for non-native English speakers". I never made the claim that someone who uses English on the Internet is likely a native English speaker. I am claiming the converse—that people who natively speak English are likely to use English on the Internet.