The latest PEW study shows continued decline in church memberships, including Catholics, Protestants and even Evangelical Protestants. The overall "Christian population" feel from 78.4% to 70%, from to 2007 to 2014. According to a short Christian Post article on the Pew findings, "The only Christian groups to register minimal growth at 0.1 percent were Jehovah's Witness, at 0.8 percent of the population, and those in the "Other Christian" category, at 0.4 percent." I wonder if the increase for the JWs is higher, since the number of JW participants in the PEW study was very small compared to their population in the US. A footnote in the PEW study notes that and gives a margin of error of plus or minus 7.
See below for the link to the Christian Post article: