It seems that the major religions growth may already be shrinking, if you look half way down this page you can see the percentages and how much they have grown or shrunk. I am proud to say that 'No Religion/Atheist/Agnostic' is the fastest growing section of the American religious demographic at +6.6%

. But the overall numbers are small relative to the big two, unfortunately.
Refer to table 'Self-Described Religious Identification of U.S. Adult Population: 1990 and 2001'
Demographics of the United States