The thing is that the enterprises seem to be okay with them being the only user of their open source projects. Don't ask me why they then decide to do open source projects in the first place, but that's what I'm seeing.
I imagine, at that scale, that opensourcing is a big win because you can have a proprietary stack, but still hire people that already know it, or won't be reluctant to invest a few years of their life into working on it. You also don't have to pay as much for educating new hires as you would for a totally closed proprietary stack, and new people faster become productive. Hiring contractors that work with that stack on a short notice also becomes easier. For a large co, these kinds of benefits might be more than enough to justify cleaning up the code enough to opensource it.
Creating and maintaining a community around an open source project is a lot of work, and it's understandable if Facebook doesn't want to pour many resources into this particular project, even if they keep it open source.
Because they sometimes get lucky, and people use it. Apple would probably have been just fine with everyone shrugging their shoulders at clang and continuing to use gcc, but clang adoption by others has been of benefit to them anyway, say.
And worst case, there's no major downside to open sourcing language tools.
Worst case is you didn't protect your IP and people get fired because the competition makes the money.
Slack came out with a blog post a few month ago, writing that they use the Hack features on top of HHVM. That is the only one besides Facebook that I know of (there are probably more)
Edit: Source for Etsy using PHP 7 now https://speakerdeck.com/wcgallego/hitting-the-turbo-button-u...
"MediaWiki 1.31 requires PHP 7.0.0 or later. Although HHVM 3.18.5 or later is supported, it is generally advised to use PHP 7.0.0 or later for long term support. "