> It powers all of Messenger.com. The team put out a case study where they went from ~10 bugs a week to less than 10 a quarter by switching to Reason

When they did that, I was running into numerous behaviour bugs on Messenger.com. Anything from weird image uploads to link previews not showing up in one chat but working in another etc.

Reduction in pure code bugs is nice and all, but a very synthetic and useless metric overall IMO.