100% of our web front end developers use Hack now. This has been an organic process of growth over the past year, by which I mean our engineers are using it because they like it and see value in it, not because there's someone standing over them with a big stick :-)

The biggest learning step for our engineering teams was to treat type errors from Hack as actual logic errors. We have a collection of "linters" that provide advice on code style and other nice-to-have factors. Some people (quite reasonably) initially thought of Hack errors as lint-like stuff that it was safe to ignore, when in fact they indicate real logical inconsistencies in code.