The American Religious Identification Survey of 2008 showed that 15% of Americans identify as non-religious. With 311 million people in the United States (June 2011 figures), that is 46.65 million Americans who are non-religious. Out of those 46.65 million non-religious citizens, roughly 40,000 are active in any given group and are part of the movement in one way or another. The rest are either silent or remain in the closet for fear of repercussions.

http://atheists.org/blog/2011/08/11/is-atheism-a-political-movement