How can this be?

I just saw a headline that said atheism has become the largest religion in the United States.  Really?  I thought atheists, by definition, didn't believe in any god.  How can not believing be a religion?  Do they congregate together and sing songs and listen to sermons about what they don't believe?  I'm sorry, it's late, I'm tired and I will have to think about this more tomorrow, or maybe not.  I will be upset if they start sending missionaries knocking on my door to ask if I've been unsaved.