>> The fastest growing group I'm told. <<
I'm not so sure of that at all... Other than being infinitely more effective at reaching out to non-religious immigrants, I can't think of why "nondoms" would do particularly well.
I mean, the thought of rejecting a church hierarchy is hardly anything new. But the Baptist churches seem to have really, really, really taken a bad hit. SBC isn't doing so bad, but some of the other Baptist conventions are rapidly disappearing.
More conservative branches of long-standing groups haven't done particularly well (Missouri Synod, Southern Baptist, Southern Methodist). And while "megachurches" continues to be a growing format for worship, it's not so much that existing megachurches are growing, as that new ones are popping up. Is that growth? Or just consolidation? What really may give some insight into the success of non-doms are smaller churches that aren't non-doms, but have rejected their denomination's hierarchy, like the Presbyterian Church of America. (And groups currently experiencing a crisis in the parent church, like the Anglicans' AIC, probably aren't too reliable of a measure either, although I would be thrilled to hear that the total membership of the AIC is throuugh the roof!)
The ones I'm talking about are those you refer to as mega churches. They merely offer a brand of Christianity that is harder to find elsewhere. Some are good, some are not. But clearly, people want something different that still stands for something.