This is the latest in a long trend. The biggest loser is mainline Protestantism. From the early 1970's to the early 1990's, it mainly lost members to evangelical Protestantism, but since then, it has mainly lost members to the "Nones". That is evident from the growth of evangelicals from the early 1970's to the early 1990's, and their slow decline after that.
The "Nones" look like they will continue to grow.
I looked into the statistical source a little more, and apparently the "No religion" category also includes people who are spiritual but not religious 37% of that same group answered believe to the question: "Do you believe in heaven?"
What this study indicates is that people are moving away from organized religion more so than becoming agnostic or atheist, which are only 4% and 3.1% respectively.
Many of these people may be closet atheists or unsure, so it's important that we try reach out to them and let them know that there is another way of looking at life.