I'm pretty sure that most big companies won't use any CSS framework, for a very simple reason — frameworks are opinionated, and opinions constrain flexibility. They set their own guidelines and develop on top of it.
For a big project, it's much better to develop your own framework as which fits your needs, and can be scaled/altered with time.
That said, it's still a good idea to use a framework for regular projects. You can take a look at this story for some suggestions: