This thread on the news.admin.net-abuse.email newsgroup suggests that changes are afoot at SORBS. Apparently the entire spam.dnsbl.sorbs.net and dul.dnsbl.sorbs.net zones have been emptied out.
Newsgroup participant Ian Manners reports that SORBS maintainer Matthew Sullivan posted about this to the SORBS-Announce mailing list:
Some of you have noticed that the DUHL (dul.dnsbl.sorbs.net) and Spam (*.spam.dnsbl.sorbs.net) zones are empty on the Rsync and DNS server. This is quite deliberate and I apologise for not posting a message about it previously.
Ian goes on to explain that he believes Matthew is "taking a break due to emotional and SORBS stress and has emptied those databases that need his constant attention."
From review of the data I track on SORBS main DNSBL zone (dnsbl.sorbs.net), it's clear that changes started taking place on July 9th. Since then, the false positive rate as dropped to zero, and the effectiveness rate has also plummeted (from around 47% percent effectiveness on recent 7 day averages, down to 14-19% effectiveness on recent daily averages). Based on the fact that dnsbl.sorbs.net is a combined zone, I would suspect that this is in line with the SORBS maintainer removing the significant amounts of data provided by the "spam" and "DUHL" zones.
Over the past thirteen weeks, my measured effectiveness of the main SORBS zone had been on a slight, but steady, decline. From 56% effective in March, to 47% effective for the week before the data changes took place. False positives (measured against the solicited mail I sign up for, of course), varied slightly but generally were around 9% week over week.
I've been reading all I can on the topic of SORBS lately (and engaging in occasionally-heated discussions with Matthew, learning more on how he runs things), with the intent of writing a more detailed review. I'm not quite ready to do that yet, and these changes suggest that I should wait and see what happens, first. Stay tuned for more on this topic in the future.