Justin.tv and Weebly run their own networks. We have BGP-speaking routers that announce our own IP address prefixes to the global internet routing table via multiple transit providers.

Your information is out of date. We outgrew EC2 a while ago.

Outgrew how?

In a couple different ways. EC2 is an amazing service, but it's not a perfect fit for everyone. We basically wanted to buy cheaper bandwidth and more bandwidth per CPU, and there's no real path for that on EC2.

From what I can tell, Justin.tv and us (Weebly) are the only ones on that list who own their own IP blocks