That looks as if the majority of the hits went directly over to the google cache links mentioned here (otherwise most likely this would be > 50k hits at least).

When the link was hitting the HN front page your site was already blocked - when the first person mentioned that the site was not reachable this HN post had about 60 points - anyway guess its the req/sec that killed you (and no cache - guess you don't have that much static content).

Have you seen many facebook referrers from people who repost HN front page stories to Facebook?