I highly doubt the HN traffic caused Steam to go down, their statistics page[1] shows how much traffic they process each hour (and it's a lot).
Considering the ammount of people visiting the store page each summer sale (well any sale), i dont think hacker news can generate that large amount of traffic.
Re:Steam hit by major security breach
by noelsusman on Monday July 27, 2015 @08:53AM
(#9954829)
It's been down off and on for the last 3-4 days.
https://isitup.org/store.steampowered.com
store.steampowered.com seems to be down!