Instagram (Facebook) ate it. Would be nice, by the way, if they could release the source code. Having such an example of a Flask app would be very useful for the community.
I believe Instagram uses Django https://engineering.instagram.com/web-service-efficiency-at-... In any case it's still a pretty popular site/app on it's own. I don't see Facebook killing it anytime soon.
Personally I like having a big codebase as an architectural reference when I'm trying to learn a new framework. The Patreon source code (Flask too) is public, by the way.