I dont think ive ever borrowed a book from the library, not after school anyway. And for younger generations who basically do everything online the library is probably even less likely to be their first thought. So i dont think someone mentioning it is bad

What's this got to do with Melbourne? /s

I think half the communication obligation lies with the person asking, eg Does anyone know where I can get book, have tried the library they don’t have it’ would tell people what the problem is and they would get more useful responses. On Facebook in particular I see so may vague posts it’s a total guessing game to figure out what the person wants, and then they get mad the responders haven’t read their mind. (But it’s highly entertaining to read the exchanges!)

It depends a bit on the community it's in. "Have you tried the library?" could be a suggestion designed to actually be helpful, or it could be a way of saying "this group is not an F%#*$ library, go and ask somewhere that's actually supposed to hook you up with books".