Would be a nice gesture, given how much $FB has benefited from running the entirety of the worlds largest social graph on top of MySQL.
* lol
They've contributed quite a lot back to the MySQL community, I thought? MyRocks comes to mind
MySQL is truly the unsung hero of FB, the whole operation would not have taken off and become a smash hit in the first place without riding atop the back good ol' MySQL.
Unfortunately, Postgres wasn't such hot shit 15 years ago. Today Postgres is AMAZING, it is truly a great day to be alive and a programmer. Now our best choices for relational DBs are Postgres, MySQL, or Oracle 21c (lol, Oracle <3's $$$ but at least Larry is straightforward and honest about it :)
FB usually open sources many projects - but I wouldn't be surprised if this never leaves FB just on practicality. Anytime there's "global data-center scale" state management thingy, it's usually deeply integrated with whatever flavor of Zookeeper/Chubby/Service Discovery/AWS/Global metadata store/GPS Clock/Intern's cron job that makes it a pain to decouple, even worse to actually test against an open source version of these stacks.
There are plenty things to shit on Facebook for, but "not contributing to open source" is definitely not one of them. React, GraphQL, Cassandra, etc. are all hugely successful and influential projects.