Religious Congregations Membership Study 2010
Statistics may not be a glamorous field of study, but it sure is useful to researchers in pretty much every discipline. If you're interested in modern American religions, you'll probably want to take a look at the Religous Congregations Membership Study for 2010.
Just as the U.S. government takes a census of its citizens every 10 years, the National Council of Churches and the Glenmary Research Center have conducted surveys of American religious bodies for the past several decades. Their latest data from the 2010 study can be found on The Association of Religious Data Archives (ARDA), and preliminary maps are available on the RCMS site for more visual learners.
Go take a look!