From the New York Times report on the Pew Study:
The Catholic Church has lost more adherents than any other group: about one-third of respondents raised Catholic said they no longer identified as such. Based on the data, the survey showed, “this means that roughly 10 percent of all Americans are former Catholics.”
Ten per cent of all Americans would be about 30,000,000 people. The second largest denomination in the United States, after Roman Catholicism, is the Southern Baptist Church (SBC), which has about fifteen million members. Former Catholics, if they were their own denomination, would be twice as large as the SBC.