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.
