Since 1972, the Social Science Research Center at the University of Chicago (with from the National Science Foundation) has been taking a General Social Survey -- and one of the things they have been keeping tabs on is religion in the United States. The charts above show the results of that research by decade.Note that this survey (as many others have also) shows there has been an explosion of growth in the United States of people who define themselves as non-religious. Those defining themselves as non-religious have grown from 6.8% of the population in the 1970's to about 19.7% of the population in 2012 -- a staggering growth, and there is no reason to believe this segment won't continue to grow in the future.
The second chart shows an interesting thing -- that this growth of the non-religious hasn't come from all religions equally. Note that the Catholics have dropped only a minuscule 2.2% over those four decades, while the percentage believing in non-christian religions has actually grown by 8.2%. It is the protestants who have lost the biggest percentage, about 19% (from 63.3% in the 1970's to about 44.3% in 2012).
I could be wrong, but I believe this is a reaction to the hatred and bigotry being preached by the fundamentalist protestants. This hatred and bigotry is far removed from what their own "savior" (Jesus) preached, and I think it is turning many young and educated people off to organized religion(and religion in general).
Here is a demographic breakdown of the non-religious community:
