Is it true that in 1975 the watchtower falsely predicted the end of the
1975 became a prophetic focal point starting in the mid 1960's. The idea was that it marked 6000 years since Adam's creation. with the next 1000 years (give or take) being the millennium.
and a great number of the membership quit..
I'm not sure if anybody knows exact numbers. The JW faith has a core of active members as well as a fringe of people who are sympathetic, but might only attend the Memorial once a year.
Events like the Cuban Missle Crises, for example, would result in greatly increased attendance, but this would drop off when the event passed.
1975 was a similar event. When the year came and went and nothing happened, attendance fell. How many true believers actually left, I'm not sure.