According to http://www.jw-media.org/people/statistics.htm, the official figures as of August 2005 show 6,613,829 active members, 247,631 of whom are new baptisms. Overall, there was a 1.3% increase in membership over 2004. But hang on a minute. 247,631 is actually 3.74% of 6,613,829. So what happened to the other 2.44%?
Well, the number of baptisms is not necessarily an accurate indication of the number of new active members, but as "publishers" usually go on to baptism wthin a short period of time (and any anomalies would average out), that shouldn't make a significant difference. If anything, given that many people become "publishers" but never get baptised, the increase in active members should be more than the number of baptisms, but we'll assume it's as near as makes no difference.
The only thing other than defection that should cause these numbers to go down is death. The annual global death rate, according to the CIA World Factbook, is 0.887% so allowing for this, the expected increase is still about 2.86%, well over twice that recorded. This would suggest that, last year, about 1.5% of JWs (around 100,000 people) stopped being active members.
Incidentally, the world's population increased by 1.13% last year, only slightly behind the JWs figure of 1.3%. When you consider that normally a child raised as a JW would be between 10 and 15 before being counted as an active member, and also the fact that 10 to 15 years ago, the world's population was increasing faster than today, we can see just how effective the preaching work really is. It allows the JWs to increase at almost the rate expected by reproduction alone.