Phabricator[0]: code review/CI solution from Facebook. My company uses it, open development has since been halted by Facebook and we're effectively on abandonware.
Flow[1]: JavaScript typing system from Facebook. My company uses it, open development has since been halted by Facebook so we're effectively on abandonware.
EDIT: React: Javascript framework from Facebook, my company uses it, and while it has its warts it works pretty well all things considered and Facebook has continued to support and evolve it over time!
For all I know Sapling is fantastic and will be developed for years to come. But personally I can't help but feel "once burnt, twice shy" (or in this case, twice burnt once shy). I'd be happy to be wrong here because ergonomics of Git are really frustrating in many places.
0: https://www.phacility.com/phabricator/ 1: https://flow.org/
React[0]: JavaScript front-end web framework from Facebook. For good or ill, the most widely-used web framework in the world.
Not to say that Facebook will maintain Sapling, but React does stand as proof that they're not incapable of carrying an open source project to the finish line.