Instagram uses Amazon... at least their images are hosted on S3. Check out this link:
http://distillery.s3.amazonaws.com/media/2011/09/17/42923def0eb141fc8bb4ab0c963d0dbc_6.jpg
Also, I'm pretty sure Reddit moved away from Amazon Cloud after the most recent outage.
Here's a link to Amazon case studies that details some of their larger clients:
http://aws.amazon.com/solutions/case-studies/