I always find it interesting when people convert to a pagan/neopagan religion like Wicca to an Abrahamic religion such as Christianity. I myself converted less than a year ago from Wicca. It took the the love of a very special girl to bring me to Christ; but it was Wicca that brought me to God in the first place. Pagan religions are actually a great stepping stone from atheism to begin to build a relationship with God. That said, it shouldn't be considered "alarming" that the religion is growing; especially considering they preach many of the same morals as Christianity. Of course it's evident from this thread that many Christians don't have a proper understanding of the religion. I blame the fact that there are many false (otherwise known as "fluffy") and young Wiccans who pervert a beautiful religion by focusing on the "magick" (love spells and the like). I'd rather Wicca be the third largest religion in America than Scientology. (to be fair, Scientologists have a strong belief in helping others, and many do this better than the majority of Christians.)
Besides, anyone who thinks Wicca is a legitimate threat to Christianity is being ridiculous. The real threats to Christianity are militant Christians like Fred Phelps and false Christians who have never read a word of the Bible or prayed.