I joined Facebook in 2019 and left this year.
Prior to FB, I worked mostly with python, javascript, and C++.
Even at FB, I worked mainly in python (instagram backend), but spent a lot of time in the Hack codebase.
My experience was that FB's Hack + HHVM stack is much easier to work with and felt more productive than any other backend stack I've used. It's important to consider that a huge portion of Facebook's backend is one giant HHVM monorepo (called www). The consistency and uniformity has allowed FB to build lots of tooling and developer productivity on top of this one stack. For example, when you add feature flags, the tooling will automatically create a diff (PR) to remove the feature flag once it has been fully rolled out for a few weeks.
There are rough edges and weirdnesses, but HHVM is pretty actively being improved. Old mutable builtins are being (or have been) removed, the type system has gotten better, better error and warning messages all the time. FB is very responsive to data on developer productivity.
EDIT:
Another anecdote, when I first joined I was working on both the Instagram python codebase and the Hack API codebase (some of Instagram's APIs are in Hack/HHVM). I constantly wondered why we didn't migrate the Hack code to python, and talked with various people about proposals and potential paths to do the migration for the APIs I worked on.
After a few months of working in both codebases, I completely flipped. After witnessing insane bugs and horrible architectural contortions designed to mitigate python performance issues, I wondered why we didn't just migrate the python codebase to Hack. Python (on cpython/cinder) is just not ready for large scale web services, and Hack is a much more productive environment than Java/C++/Rust for backend.
Typescript may be an even better option, but has some issues of its own.
Yes. Instagram doesn't use the Django ORM though, and I believe they built their own async view implementation since it existed before Django went async.
IMO Typescript will grow on the backend. It's mainly just not as mature as alternatives and will get better over time. There are small problems similar to python though, like module-level mutable global state. That interferes with certain tools and fast restarts, but can be mitigated the same way as is done in python, ie https://instagram-engineering.com/python-at-scale-strict-mod...
Go is not dynamic enough to build useful abstractions. It does not allow developers to make themselves more productive. It's also more difficult to build hot reloading and high quality reflection based debugging.
Your second question is based on an incorrect premise. Hack is internally built, but it is also open source:
https://github.com/facebook/hhvm
I joined Facebook in 2019 and left this year. Prior to FB, I worked mostly with python, javascript, and C++. Even at FB, I worked mainly in python (instagram backend), but spent a lot of time in the Hack codebase.
My experience was that FB's Hack + HHVM stack is much easier to work with and felt more productive than any other backend stack I've used. It's important to consider that a huge portion of Facebook's backend is one giant HHVM monorepo (called www). The consistency and uniformity has allowed FB to build lots of tooling and developer productivity on top of this one stack. For example, when you add feature flags, the tooling will automatically create a diff (PR) to remove the feature flag once it has been fully rolled out for a few weeks.
There are rough edges and weirdnesses, but HHVM is pretty actively being improved. Old mutable builtins are being (or have been) removed, the type system has gotten better, better error and warning messages all the time. FB is very responsive to data on developer productivity.
EDIT: Another anecdote, when I first joined I was working on both the Instagram python codebase and the Hack API codebase (some of Instagram's APIs are in Hack/HHVM). I constantly wondered why we didn't migrate the Hack code to python, and talked with various people about proposals and potential paths to do the migration for the APIs I worked on.
After a few months of working in both codebases, I completely flipped. After witnessing insane bugs and horrible architectural contortions designed to mitigate python performance issues, I wondered why we didn't just migrate the python codebase to Hack. Python (on cpython/cinder) is just not ready for large scale web services, and Hack is a much more productive environment than Java/C++/Rust for backend.
Typescript may be an even better option, but has some issues of its own.