> Are there any large Hack codebases I can explore?

depends what you mean by 'large' - perhaps https://github.com/hhvm/user-documentation ?

> Does anyone know if FB tracks changes to PHP so Hack is "up to date"?

No, for the most part, Hack no longer considers PHP 'upstream'. Exceptions are things like security fixes to extension functions, if that particular extension function was derived from PHP.