Here’s a link to an article on the move to Christianity in Indonesia:
http://www.time.com/time/magazine/article/0,9171,1982223,00.html
Hard to get a good handle on facts, especially in a place like Indonesia, with its many, many islands, languages and peoples. However, let's think this through.
A high-side optimistic estimate would be one million conversions from Islam to Christianity per year. (Conversions don't all come from Muslims; they come from Animists and even from Chinese Communists in Indonesia, too.) The population of Indonesia is around 250 million, about 80% Muslim, 10% Christian, and 10% another including Hindu, Confucist and Animist. So, say 200 million Muslims. If one million convert from Muslim to Christian per year, that makes the annual conversion rate 0.5%.
Speaking carefully, five tenths of one percent is not what I would call massive.
(I am chastened when I think of a handful of people in my own family who have in practice exited Christianity. I would bet that many Freeper families know the sadness of seeing many -- well, use the term "backslidden" family members.)
I could well be wrong. I want to be wrong. I am open to the Holy Spirit's surprises.