Also, this report only lists the top 25 Christian churches size wise. I’ve read other reports that the 7th Day Adventists are actually the largest growing denomination percentage wise, but they aren’t large enough yet to make the top 25. Other reports list Wicca as growing very fast percentage wise, but that would never be counted in this report because it’s not a Christian religion.

BYU professor David Wright has a project trying to figure out accurate LDs statistics. He claims that only 33-35% of the total Mormon membership is actually active and practicing. So if you went strictly by adherents, that would move total numbers to 4-4.5 million. He says about half of those are in the US, so 2 - 2.25 million US active adherents. You should be able to easily find references to his work to verify. I don’t currently have it handy.

I think the information to be focused on, for your purposes, may be comparing growth rates of the LDS over the past 20 years. In the late 80s the growth rate was 8%, in 2000 it was down to 3%, now it’s down to 1.56%. And that’s with counting everyone, regardless of whether they are active or not. There could actually be a negative growth rate if only the numbers for the active adherents were used.

It seems more logical to focus on ways to keep the members the church has acquired, as opposed to wasting time on concerns of what a ranking might be, especially since because reporting methods vary, the reports really are not accurate. Like I said, only if every denomination used the same reporting method would there actually be an accurate comparison.

A couple links talking about church sizes/growth and reporting methods I have offhand:

http://www.religioustolerance.org/chr_prac2.htm

http://www.washingtonpost.com/wp-dyn/content/article/2008/02/04/AR2008020401526.html