If it isn't asking too much, would you mind telling HN about it? what kind of traffic you saw, what hardware/software platform you have in place?
I think the hardware platform is not really interesting, as the DDOS was only caused by the worker pool of Apache2 being too small. Each hit of the site creates about 30 additional request for css,js,png and gif files. So 9000 hits in 3h mean something like 270.000 requests in 3h. Apparently this was too much for 50 workers.
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?
I'm pretty sure that the amount of req/sec caused the problems. After a configuration change between 150 and 200 apache2 workers were constantly occupied and neither network ,cpu or ram were even closely used to their capacity. (Before the change we had a hard limit of 50 workers.)