Only once to my knowledge has a substantial group left the CoS for example in recent times, when some Scientologists left the Amsterdam Org together and set up a Rons Org.
Scientologists left the 'Church' en masse following the Mission Holder Conference. Enough so that the picket in front of the Big Blue Complex may *still * be the largest single picket against the 'Church'.
The Cult didn't even begin to recover until '90 or so.
Zinj