There are 8 billion people on this planet, nothing happens instantly.
Facebook took a long time to spread around the globe. Same for reddit, this is a quote from the Wikipedia article:
As of August 2024, Reddit is the 9th most-visited website in the world. According to data provided by Similarweb, 51.75% of the website traffic comes from the United States, followed by the United Kingdom at 7.15% and Canada at 7.09%.[6]
More than two thirds of reddit traffic still comes from Anglophone countries to this day, and that percentage was surely much higher back in the early days.
I think you're severely overestimating how many people from other countries actually use Western social media. Between the language barrier and the technology barrier, most people on this planet simply don't have any opportunity or desire to use a site like Reddit or Lemmy. Facebook has slowly but steadily made global inroads, but by the time it got popular in non-western countries, Americans had largely moved on.