Not really a comparable company. Quizlet has <100 employees and $32M funding. Slack has <5000 employees and has a $15B market cap on the public market.
I'm sure there are a ton of small-ish VC-funded startups using all sorts of technologies like Hack. As far as big companies though, FB and Slack are pretty much it.
True. My understanding is that they're fairly active in the Hacklang community; they're big enough to have received a mention in the HHVM docs: https://docs.hhvm.com/hhvm/FAQ/faq
