This more or less reflects what I have been seeing (on a low volume private mail server) but the dropoff for me started around late October with connections staying at relatively normal levels but the amount of spam coming in falling to almost zero.
Curiosity (i.e. tcpdump) turned up the reason - what looked like a spammer 'template typo' that was causing emails to fail relatively silently with a protocol error, so they never got far enough to be counted as anything. It looks like they have fixed it now and are trying to make up for lost time.
I don't know which botnet was responsible so I couldn't say how widespread this was but I don't think it's entirely coincidental.