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).

1. http://store.steampowered.com/stats/content/

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.

It's been down off and on for the last 3-4 days.