Well, yeah, but it's not anecdotal. There is data to tell you how big Facebook is and was outside the US, in what territories and by how much relative to their US popularity at what point. My personal experience just happens to match those numbers (India, by the way, is Facebook's current biggest market).
I would also point out that by your own data, which is accurate as far as I can tell, 49% of Reddit is not American, so even with its more US-focused audience the assumption that users are American unless proven otherwise is wildly ethnocentric.
Now, I agree with you that assuming things about anonymous accounts, and especially anonymous accounts writing in English, is foolish. Lots of people are fluent in English who are not native speakers and definitely who are not from the US. Most, in fact, depending on how you define your parameters.
I disagree that this is "human nature", though. I don't assume the same thing from people who speak my native language online. I also don't assume the same thing about English speakers. The reason the OP is asking is that US ethnocentrism stands out. That's not to say it's not natural. We non-native dwellers in anglocentric social media will often comment on US cultural and political minutia, because US cultural and political minutia is present and relevant to us in a way ours isn't to Americans (thanks for that, cultural imperialism). We pass for Americans in more situations than some American lurking in a German-language forum would, and we're likely many times more numerous than... well, Americans lurking in German-language or Chinese-language socials.
But it being natural doesn't mean it isn't notable or an issue or a symptom of a dysfunction. Which it is, and it does annoy me for that reason.