Hi!
Other tools we are using, such as Phabricator, will also be following HHVM to Hack (presumably).
I am not sure this is the case. Mozilla recently declared they want to use Phabricator[1], but I heard no mention about HHVM. Which makes me think that one stays on PHP. Also, Phabricator is now independent from Facebook, afaik, since it's developers have separate company, Phacility.
[1] https://wiki.mozilla.org/Phabricator