It seems to me that there is a flipside to this. Sure the number of people who are not religious is going up but of those who remain religious there is a growing number who are extreme in their belief. 45% of Americans believe the world is only 10,000 years old, or less. 60% believe Noah's flood actually happened. And to borrow a line from The Newsroom, America really does lead the world in the 'percent of adults who believe angels are real.'
Another interesting factor in that graph is the slow but sure rise in 'other'; alternative and non-Christian religions are on the rise. Perhaps Cosmism and Universalism will become significant, or even Pastafarianism.