I think that we should share things if we want to. I don't think that in any manner of fashion it is a DUTY to do it. Of course we can feel moraly obligued to tell someon they are doing something 'bad', but we can shut up, and let whatever hapens hapen.
It is fun to talk about ideals with others, it is always fun to do some convincing. But it is not required of us, and it does not necesarily have good outcomes. But whatever, we must all do what we feel is right.