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...