I doubt anyone here is working to 1% of fb scale, let alone 10%.
The answer to "why Hack" needs to be viewed in the historical context of "when Hack" and what was happening (or not) in the php ecosystem at that time.
Things have changed a lot since, in terms of performance, language longevity, ecosystem etc. Its a perfectly reasonable language to adopt for many orgs.
More specifically, Hack was developed during the time when it looked like the PHP project wasn't going anywhere (it was stuck at version 5.x for ten years while version 6 was in the works, then abandoned, then version 7 was developed based on version 5 and finally released in 2015).
Given that Hack had generics as early as 2014, it seemed that those who were worried about PHP being stuck were right.
Unless I'm misunderstanding what this proposal (not finished artifact) is about.
Some people here work at (exactly) 100% of fb scale.