http://www.pewforum.org/Unaffiliated/nones-on-the-rise.aspx
The number of Americans who do not identify with any religion continues to grow at a rapid pace. One-fifth of the U.S. public – and a third of adults under 30 – are religiously unaffiliated today, the highest percentages ever in Pew Research Center polling.
In the last five years alone, the unaffiliated have increased from just over 15% to just under 20% of all U.S. adults. Their ranks now include more than 13 million self-described atheists and agnostics (nearly 6% of the U.S. public), as well as nearly 33 million people who say they have no particular religious affiliation (14%).
This large and growing group of Americans is less religious than the public at large on many conventional measures, including frequency of attendance at religious services and the degree of importance they attach to religion in their lives.