Ask HN: Why is EC2 more often down than Amazon?

EC2 seems to have trouble sometimes, like recently, with one or more data centers or central infrastructure down. This leads to company websites and large parts of the startup world being down even for those one thinks are multi-data center and know their stuff like Foursquare.

Why is Amazon (never?) down? Do they use other hardware? Is Foursquare and like not as good in designing failsafe setups as one thinks?