I worked on Hack during my Meta bootcamp and was surprised to learn that many people used and relied on Hack outside of Meta. It didn’t appear to be anywhere near the size and scope of PyTorch and React but the number of users looked relatively large!

Alternate anecdata, I've never seen someone use it outside of Facebook/Meta. I'm sure some people did but PHP 7 solved the performance issues people had with PHP 5, and PHP 8 nuked some of the footguns people complained about all the time, to the point, the few shops who looked at Hack / HHVM (and weren't switching to another language entirely) just stayed on PHP.

Not sure of the status today, but Slack is/was one of the most high-profile non-Facebook users of Hack, e.g. https://slack.engineering/hacklang-at-slack-a-better-php/

The post is from four years ago however, does anybody have an update on that topic?

Slack has a few eng blog posts from last year which seem to indicate they still use Hack, among other languages.

https://slack.engineering/hakana-taking-hack-seriously/ describes their open source enhanced Hack type checker, which is still actively maintained (most recent commit yesterday).

And https://slack.engineering/slacks-migration-to-a-cellular-arc... mentions they use a mix of languages: "services in the user-facing request path are written in Hack, Go, Java, and C++"

As of the beginning of this year, slack still used a lot of hhvm.