Rik van Riel wrote:
A lot of the variation in PSBL zone size seems to be due to both random variations in spam volume, as well as law enforcement shutting down botnets. Whenever a big botnet has been shut down, spam activity tends to be noticably less than before. I expect email spam is down simply because the spammers have also found alternative ways to spam, eg. click hijacking and sharing of spam material through social media.
This is an update to a similar chart from last year.