If a publisher stops reporting it's only after 6 months are they considered inactive, so the stats are misleading. Once I had left I went back after a few months only for a few weeks though. This is what can be termed as yoyo publishers. I am Df'd anyway so I wouldn't have been counted as a publisher, but my attendance would have been counted.
There is no real way to find out how many have left, but we can see just from JWN the newbies posting there is a high volume of those leaving.
Kate xx