I imagine they use a few different CDNs for things like this.

That doesn't appear to be the case does it? Amazon sites are all working fine, at least for me

Their CSS and JS were down for a few minutes. I was able to login to Amazon but the entire site was in Times New Roman, but was fixed a few minutes later

Must be more than fastly. Heroku is also down.

That's also because of fastly, I've got this response from the Heroku dashboard:

Fastly error: unknown domain: dashboard.heroku.com.