[throwaway2016a]

I'm a little surprised HHVM is still around, last time I checked it was barely (if at all) faster than vanilla PHP and PHP now has a lot of the comparible features like progressive type safety and concurrency (to an extent).

There are even competing PHP compilers/interpreters/runtimes now like there has been for a while with other languages.

I'm guessing there is some benefit to running HHVM on the type of large scale deployments Facebook has or there wouldn't be a need to keep it around.

[conor-]

Wouldn't the large amount of existing services written in Hack be reason enough to keep it around?

The cost of rewrites at that scale certainly has to be far more expensive than the cost of paying a team to maintain HHVM

[throwaway2016a]

Generally yes but the languages are similar enough that the refactoring could be automated. Of course, that would create a fair amount of QA and testing work but if you have good test coverage you can mitigate that.