There's a big drop off, which would indicate an algorithm change by someone (statcounter, google?), but also a slow decline since then. Maybe Jeff has been busy with StackExchange and Discourse and hasn't been posting as much?
I think its most likely google due to google algo change (after doing some research). For big sites like CH, even if you don't update your sites for a long time, you are not likely to get such a dramatic traffic drop.
I remember his sites having text based ads, which might have something to do with it.