The event was actually on 2012-06-06 and it was invite only. The event was mostly about the infrastructure of Facebook. It was actually done quite tastefully with a main speaker giving an overview of the whole system. Luckily they did not turn the event into a recruitment event which was quite pleasant.
Unfortunately one of the posters that I had actually want to see and chat to the engineers involved wasn’t there, there were enough engineers around for me to harass with questions.
This event was also the first event that I have been to in Dublin which required an NDA, that was sort of strange. Some people that I know didn’t attend as a result of the NDA.
All in all Facebook does have some pretty cool tech internally (like all the other big dotcom companies). Some of it is even opensource!