Remember the Time magazine article published about "JW churn"? This thread and the chart by splash is more corroborative evidence of that.
The Time magazine article states this about JWs:
An even more extreme example of what might be called "masked churn" is the relatively tiny Jehovah's Witnesses, with a turnover rate of about two-thirds. That means that two-thirds of the people who told Pew they were raised Jehovah's Witnesses no longer are — yet the group attracts roughly the same number of converts. Notes Lugo, "No wonder they have to keep on knocking on doors."
Full article here:
http://www.time.com/time/nation/article/0,8599,1716987,00.html