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.
And you also have here a direct download of the full leak, including the database:
https://archive.org/details/B93B1C2921EFF0846DA268968755D01B...