I guess the question is, what do you mean by "massive growth?"
How many pagans are there in the US?
I don't know that? Is the ACLU Pagans? are homosexuals that are trying to enforce there values on people that follow the Holy Scriptures Pagans? Pagans are idol worshippers and I guess they can manifest there forms of worship in many ways that would be considered paganism...
I have also read that many people around the globe that are dis-illusioned with the Bigggest Mainstream Religions of the world:
Christianity, Judiasm and Islam are beginning to think that they have failed, they bring no PEACE and only cause wars and dis-harmony in people's lives, saying that these people would be easliy swayed into Paganism.
"How many pagans are there in the US?"
Counting just the ones in the Episcopal Church, seems there are a couple million.