Although church attendance among Protestants has remained stable, Mass attendance has declined steadily from the 1950s, when 75 percent of Catholics went to church every weekend, to now, when only one-third attend weekly. This decline has been steeper among younger cohorts (only about one-quarter of todays young adults attend weekly) than older ones (about 60 percent of whom still attend weekly).
http://www.bc.edu/bc_org/research/rapl/church-in-america/davidson.html
Protestant church attendance has ALWAYS averaged about a third of registered members.