The shifting SBL, as experienced here
I still sort of run a mail server which gets a low enough connection volume that I can monitor the logs directly. This MTA rejects connections from SBL listed IPs, at a sufficiently low volume that I almost always look into the actual SBL listing (partly because I may want to apply my own blocks, including IP-level ones).
In the beginning, the volume of SBL hits was low but most of the actual SBL listings were for network ranges (not just single IPs) owned by what I privately characterized as 'the worst of the worst'. These were the people and organizations who spammed so many people so often that they finally convinced the SBL that they were very definitely dirty. Hits were rare partly because there never were really large numbers of these people, partly because I and other DNS blocklists blocked such people before the SBL, and perhaps partly because these people just didn't target me very often.
(I and a fair number of other people felt that the SBL was far too conservative and gave spammers way too many chances, but the SBL had its standards and that was it.)
I'm not sure when things started shifting, but this is not the pattern that I see today. The modern SBL experience is that most SBL hits are from single IPs that are listed as probably compromised or, to a lesser extent, from IPs that are on the SBL CSS. Hits from genuine SBL listed dirty blocks seem to be rare.
Out of curiosity I pulled eight days of records from the department's main mail gateway and looked through them for SBL rejections. Of the 80 IPs that (still) had SBL listings, the SBL CSS accounts for 35, 188.8.131.52/24's SBL136747 listing for four, and a random sampling of everything else shows single (compromised) IPs.
(Yesterday is a bit different. There are 27 IPs that are still SBL listed, with 21 of them on the SBL CSS. But two of the remaining were for bad netblocks and one IP was listed for spammer hosting. The other three were the usual single compromised machine pattern.)
I don't know what this means, if anything; I just find it interesting.
(I can come up with all sorts of potential theories but I will spare you all; they're generally obvious anyways. Just in case there's any doubt, I should note that I'm all for the SBL listing all sorts of spam sources and so I have no objection to the apparent new inclusion of compromised machines that are spewing advance fee fraud and phish spam and so on.)