How common is giardiasis in the us?

Pretty common. Estimates for symptomatic or asymptomatic carriage range from a few % to 1/20 in the US. Carriage is higher in various populations like infants and those in institutions.
Very common. Widespread in wilderness areas, and statistically probably present these days in most mountain streams and lakes. Also, clearly almost ubiquitous in the feces of canadian geese, who are literally throughout the western states.