The Pew Center has some interesting research about "religious unaffiliation."[1] Unaffiliated is the second largest religious identification by a large margin at 16%, Judaism is the third largest group at 1.8%.[2]

The Global Religious Landscape Report has some interesting figures for the world population. Religiously unaffiliated was the third largest reported/identified group in the survey at 16.3% just barely nudging out Hinduism at 15%.[3]

[1] http://www.pewresearch.org/topics/religiously-unaffiliated/ Growth in US Specifically http://www.pewforum.org/2012/10/09/nones-on-the-rise/ and http://www.pewforum.org/2013/07/02/growth-of-the-nonreligiou...

[2] Page 50 http://www.pewforum.org/files/2014/01/global-religion-full.p...

[3] http://www.pewforum.org/2012/12/18/global-religious-landscap...