@scottalanmiller said:

That Facebook is using HHVM in production makes me a little more comfortable with it.

True, but I would guess that Facebook's application runs mostly on code that Facebook is responsible for maintaining. On the other hand, most WordPress websites have at least a couple of third party plugins and/or themes and those usually assume you're running on PHP 5 so there's a higher chance of stuff like this happening.

Probably wouldn't be an issue very often but I would be more paranoid about updates breaking things if I were running HHVM in production.