I remember reading somewhere that Buddhism was growing very fast as well. I can attest to this when I attended a Buddhist temple and saw the group grow overnight. But it's hard to measure as a lot of people who attend are 'seeker' types who rarely self-identify as Buddhists.
I would say that the growth of Islam can be largley attributed to immigration, at least from what I've seen.
Oh yeah, I agree that atheism is not a religion. It's not like atheists go to the atheist temple and light incense to the atheists saints or anything. There were times in history where atheists adopted rituals in movements, and some of the 'new atheists' are almost fundamentalist-like in their zeal. But I don't think this means they consist of a 'religion'. It's a philosophical system rooted in materialism - that all phenomena can be explained through observation of the physical world. Hardly the stuff of a 'religion'.