As someone who is going to school to become a library technician and who works at the university's library part-time... We don't ever just walk up and start chatting to students while they are studying.. especially in the designated "quiet zone". Our library has two floors, the main floor is a modern styled "working library" where people can be in groups and talk freely amongst themselves, while the other floor is the quiet study zone, where no loud conversations happen, only whispering is allowed if you need to talk.
But never have I walked up to or seen another librarian just walk up and start talking to students who are working, nor do we "shh" people, we usually just tell people to lower their voices :D