I've always thought that a meaningful statistic, that isn't specifically reported by the society is the percentage of bible studies that get baptized. Please keep in mind that many of those studying are children as are many of the ones being baptized.
I realize that individuals may study for more than one service year (especially a witness child), but over the course of a decade or more the percentage of bible studies to baptized has shown itself to be an indicator of health for the society. There is a steady downward trend of those studying getting baptized.
Here is the U.S. and World statistics since 1990 for the number studying vs. the number baptized:
(honestly I can't figure out how to format this stuff very well)
Percent of individuals baptized vs. studying
Year u.s. world
1990 9% 8%
1991 8% 8%
1992 7% 7%
1993 7% 7%
1994 7% 7%
1995 8% 7%
1996 8% 8%
1997 10% 8%
1998 9% 7%
1999 8% 7%
2000 7% 6%
2001 - 5%
2002 6% 5%
2003 6% 5%
2004 6% 4%
2005 6% 4%
In the last 15 years they went from converting 9% (U.S.) and 8% (world) bible studies into baptisms down to 6% (U.S.) and 4% (world).