From my Hack post:
Hack is also vulnerable and politically unstable. It’s developed and supported exclusively by one huge web company, and they may decide to deprecate and replace it in a few years as their technical needs change or its core engineers move on to new projects.
Many Hacker News commenters and some human beings asked why this was any different from Microsoft effectively owning C# or Apple effectively owning Objective-C.
Microsoft and Apple have massive vested interests in supporting their languages and platforms. They stand to lose a lot to their core businesses if they stop. Developers’ interests align somewhat with theirs in this regard: one developer doesn’t have a lot of power in those relationships, but the sum of all developers definitely does, so these companies generally need to care for these languages and maintain these platforms for a long time.
Facebook has no reason to care about HHVM or Hack except that they use these tools themselves at the moment. They get almost no benefit from anyone else using them,1 and they will suffer effectively no cost if they kill them, ruin them, let them stagnate, or take them private again. Their incentives don’t align with outside developers’ interests at all.
(The same applies to some of Google’s projects, like Dart.)
Except finding bugs and occasionally submitting patches, but Facebook is so massive with such a large codebase that they’re likely to find most bugs relevant to them long before anyone else does. ↩