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: