Most of those companies use a bunch of technologies, Scala being only a small part of their stack. Only a few of those companies use Scala as their main language.

Is it really common for companies to have a single dominant language though? It seems that most companies have their eggs in four or so baskets.

Yes, totally. I'm talking about their core language. LinkedIn's core language is Java. Google's main languages are C++ and Java. Pinterest is mostly Python. Quora's core is Python. Stripe's core is Ruby.