I did check statline.nl; a database of statistic information about Netherlands.
They told 4.639.000 Dutch people went in 2003 by plane on holiday. Total Dutch population is 16m, so that makes 29% of total Dutch habitants a year only for holiday. Count in all those other people traveling for anything. And of course: all Dutch post-high school people go a year abroad, to ..... Australia.
Dutch population is 16m and only AMS
gets 44m pax each year. But, this includes all transit-pax. On the other hand: many Dutch people near the borders of Germany and Belgium can use airports in those countries. And you must count-in relatively small, other Dutch Airports like Rotterdam/Eindhoven/Groningen/Maastricht. Moreover, in Netherlands live many immigrants (in biggest cities near 40% right now).
And still, this is only EVERY year. Those 80% isn't very hard to get!
When comparing Netherlands - USA I would like to make some remarks about 'rich'.
In USA the %-population of 'poor' people is MUCH higher than Netherlands. But also: in USA is a (small) number of people who live in excessive wealthy with private planes etc. This kind of wealthy is in Netherlands -indeed- not such common. But this does not count really when looking for a % of populations who did ever fly on a commercial airplane. On the other hand, Netherlands does have almost none domestic flights. All ('poor') people find work in this small country and don't need to go thousands of kilometers by plane to find work or use plane for any other essential reason.
My point? Aviation-penetration in Europe is not else then USA! Number of flights is probably lower, due to USA domestic flights vs. EU trains.