Because Amazon doesn't want to confirm it I'm pretty sure they don't use ECC.

bbgm · on July 16, 2013

(My team owns product management for the Amazon EC2 instance platform)

I wanted to clear up all the confusion on this topic and am cross-posting on other threads where this has come up recently.

All the hardware underlying Amazon EC2 uses ECC memory. In our experience ECC is a necessary requirement for server infrastructure. We will be updating our detail pages/FAQs with that information.