Oh I did this with the Facebook one and redirected them to a 100MB file of garbage that is part of the Cloudflare speed test... they hit this so many times that it would've been 2PB sent in a matter of hours.

I contacted the network team at Cloudflare to apologise and also to confirm whether Facebook did actually follow the redirect... it's hard for Cloudflare to see 2PB, that kind of number is too small on a global scale when it's occurred over a few hours, but given that it was only a single PoP that would've handled it, then it would've been visible.

It was not visible, which means we can conclude that Facebook were not following redirects, or if they were, they were just queuing it for later and would only hit it once and not multiple times.