What percenatge of the us population ski or snowboard?

That depends on which country you look at.

In Sweden the lift organisation have statistics saying that 28% of the population buy a lift pass for at least one day (most go a week) every year. To this you have have to add all the cross-country skiers which probably are just as many or more but of course many of them are in the lift pass buying group.

I assume these number are pretty representative for countries with skiing traditions i.e. Sweden, Norway, Finland, Austria, Switzerland etc.