As much as I'd love to jump on the "stupid Americans" bandwagon, this seems to be a big problem not only in America.
After the reddit exodus and before I had a good setup for lemmy, I used Facebook for a short period. Most of my stuff there is from US, UK and Norway, and the number of people in the comments who can barely put together a coherent sentance is astonishing. Far below 6th grade level by any standard.