"A 2001 survey directed by Dr. Ariela Keysar for the City University of New York indicated that, amongst the more than 100 categories of response, "no religious identification" had the greatest increase in population in both absolute and percentage terms. This category included atheists, agnostics, humanists, and others with no theistic religious beliefs or practices. Figures are up from 14.3 million in 1990 to 34.2 million in 2008, representing a proportionate increase from 8% of the total in 1990 to 15% in 2008. Another nation-wide study puts the figure of unaffiliated persons at 16.1%."
The actual numbers are 8.2% to 15.2% but are rounded to the nearest whole number.
2008 minus 1990 = 18 years
15.2% minus 8.2% = 7%
2080 minus 2010 = 70 years
70 / 18 = 3.89 cycles (rounded up to the nearest hundred decimal)
3.89 * 7% = 27.23 percent more
27.23% + 15.2% = 42.43%
70 years from now if the percentage increase continues at its current state and does not rise or lower, the U.S. will have a 42.43% of non-religious society.
Now let us take into account that also other religions are on the rise from Christians leaving Christianity into Judaism and Islam as well as Hinduism. So these numbers are also taking away from the christian majority. Also those who lose their faith becoming Atheists this is also taking away from Christianity. Then take into account that with more publicity and acceptance of Atheism more and more people will be comfortable with stating their inability to accept logically fallacious claims of religion. So it is more likely that the growth percentage rate will increase. As it has been on the rise in the past 30 years.
I'd say the figures are more than likely to be closer to or even above 50% nonreligious in the U.S. by 2080. The numbers are speculative, but since the numbers are based on historical facts they are reasonably more likely a true projection over anything anyone else can offer and if you look at the math from the American Physical Society the patterns are similar to my math projections. Link below.