The longest thread I recall was my "Another Piece of the Puzzle" thread started in the fall of 2006, not surprisngly about Merion. At one point the thread was over 50 pages long.
The amazing thing about the thread is that it is now only 29 pages, at one time it was reduced from over 50 pages to the mid-twenties. At the time one could delete their post as if it never happened.
Around the time Tom MacWood left the site, the attention of some uninvolved posters focused on the behavior of Wayne and TEPaul toward MacWood, me, and others who disagreed with them about Meiron. This prompted Wayne and TEPaul to delete the posts even they recognized as being rude, offensive, or otherwise out of line. They did not delete all their posts, just the egregious ones. This amounted to the deletion of between 600 and 700 rude, offensive, or out-of-line posts in a single thread.
The Merion bunker threads were quite long, as I recall. I don't think I was involved in them.