My site has been front-paged on HN several times as well, and I can corroborate. On those occasions my log files show something like a tenfold uptick in average traffic, while GA registers a tripling or so. When I get front-paged on reddit, by contrast, GA shows about 70-80% as many visitors as indicated in the access log.

Also, my site is supported by donations, and when one of our articles gains traction on HN we get a substantial increase in one-time donors during that day. Other big sites like reddit barely bump the numbers at all, despite often bringing a much greater increase in total visitors.

One time we were front-paged on /r/TodayILearned, and GA showed a peak of 6,680 concurrent visitors. Hotchie motchie.