Has anyone used Google protocol buffers for anything? Do people still use them or has the world moved on?

I've used them. They are nice for cross-language serialization. However, if your language has support for something native, then that native thing is probably better. C++, for example, does not, so for it protocol buffers are very useful.

There are also a number of alternatives to protocol buffers, but the general idea is the same.

Also, these sorts of things often come with an RPC API, so if you're into making servers, it's a bit more useful even for languages with native serialization support.