That comment got downvoted to -1! I'm not saying Hack is obscure and used only by Facebook - but it would have been had Facebook not open-sourced HHVM and tried to promote its use.

It's not that obscure. Anyone with an ounce of experience in PHP can start writing solid Hack on day one. And that would still be true even if Hack were never seen in the wild.

Unless you think it's hard changing the open tag from <?php to <?hh :p