Afaik Etsy and Wikipedia only used HHVM for the performance win and not the Hack language on top. Since Wikipedias tech stack "MediaWIki" is heavily used by PHP companies it can't be using Hack features.

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...

Hack is going to be deprecated when it comes to MediaWiki, they're recommending PHP7.

"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. "