From internal communications, it looks like it was mostly a matter of some expensive but unnecessary request filters being turned on. It's not surprising that these would have caused a problem with a high volume of traffic.
We need to get better at operating our own site. In the past it's been a severe case of the cobbler's children going barefoot: few if any resources were allocated to ad.com, which was run mostly as a hobby by whoever had some spare time. There's a staff now, and they've made a lot of improvements. I'm sure scaling issues have just been bumped up on their priority list. Scaling really isn't about what software package you install anyway: it's about diligence, careful planning and hard work on the part of the team building and operating the site.
