Zuckerberg's bio on Wikipedia says that he is the son of a dentist and a psychiatrist. I'm sure there are plenty of Americans with wealthier or more connected families, and none of them have succeeded with their startup the way he has.
Sure, but only about 5%. The median salary for a dentist and a psychiatrist adds up to $333,000 which is the 95% percentile for household income today. A verrry long way from average.
So, really we are talking about the top 1%.