There's an argument that the very low numbers in Europe are a result of the different denominations being discredited over the last 400 years' political involvement. That could, in part, be what's happening here.
The Southern Baptist are hemorrhaging young people and women, for example.
Another interesting finding showed up in the Pew and recent PRRI polling. The only growing segment are Evangelicals who don't actually attend church, i.e. those for whom it's pretty much purely political.