A couple of points for those who want to compare "apples to apples"...
JW's don't practice infant baptism, so those born into JW families aren't counted as "members" in any statistics... While other religions may count baptised infants as "members", JW's don't.
In order to be counted as a "member", a person would have to go through a thorough Bible training program, get baptised, and be regularly active in the preaching ministry. A person who doesn't regularly participate in the preaching work would not be counted in any statistical number.
The quote mentioned "those raised as JW's who no longer are". Please don't misinterpret this. This doesn't mean they ever were JW's. Being born into a JW family doesn't make you a JW... (unlike other religions). A typical child wouldn't become a JW ( by becoming baptised and active in the ministry) until their mid-teens. There are certainly a number of teens that choose not to become active in the preaching work or choose to live a moral life based on Bible principles, so they would not be statistically counted.
Lastly, Jehovah's Witnesses adhere to a strict interpretation of Bible morality. Because of this, each year 40,000+ of the 6,500,000+ active members are removed from the congregation and are also not statistically counted.
In summary, if Jehovah's Witnesses were like other religions who:
1) count baptised infants as members
2) don't require any specific activity to be counted as a member
3) didn't expel non-repentant practicers of immorality
the numbers would certainly be much higher.
Here's a inside look at Jehovah's Witnesses world-wide preaching activity that has contributed to us being the fast growing religion:
, Part 2
, Part 3
, Part 4
, Part 5
, Part 6