I doubt anyone actually thinks it is a dichotomy. Everything I've seen on the subject just uses the terms "introvert" and "extrovert" as references to the extre

I doubt anyone actually thinks it is a dichotomy. Everything I've seen on the subject just uses the terms "introvert" and "extrovert" as references to the extreme ends of the scale: the 10% of people most extroverted and the 10% of people most introverted. I think that is it a scale of introversion/extroversion, with probably a normal distribution of "average" people who can deal with more social activity than "introverts" but can't take quite as much as "extroverts."