On Marvin's thread about the Weird Photo he points out that in the year Spring 1927 to Spring '28, Memorial attenders dropped by over 80% !
This was due to the failed 1925 predictions, about which Rutherford admitted "I rather made an ass of myself" (I presume he was saying "arse"),
we are not in the same position now, there is not an actual date given by the WT, 1975 was a lesson to them, and we also now have a body of people, 7Mill approx. who are used to Doctrine change, and used to failed expectations, all of their JW life this has gone on, as has the relentless mind control that the GB/FDS are being used by God whatever rubbish they spout.
The Bible Student still associating with Rutherford around 1925 was a bit of a thinker, so many left. Today's JW's have either never had, or have had taken away, any Thinking skills.
I do think though that the 100 year mark reached in 2014 will start some beginning to wonder, and a year or two after that we may see a spike in the number leaving, but I do not forsee a mass exodus, lets face it, any who are mind controlled enough to still be in it now are not that likely to wake up.
The Internet/Information age coupled with the general lack of interest in religion will hamper their efforts at gaining new members, I already think this is happening, and the WT will slowly die anyway by attrition even if no more than usual wake up and walk.