I think this is characteristic not of Americans and not even of terminally online Europeans, but rather of reddit users specifically (and heavy users of other systems that operate similar to reddit, or which had very biased moderation for a long time but no longer do, such as twitter.) Reddit is all about forming the illusion of overwhelming consensus; an opinion shared by 49% of users can be ruthlessly suppressed by the 51%, downvoted below zero and hidden from view. The fundamental nature of reddit's comment system encourages this warped perception of people who disagree with you being unreal.