Oh, I agree. But specifics differ, depending on who you ask. An important question is what we mean by “letting people know”. For example, Edgeryders has a well thought-through netiquette page. It was re-written several times, and ultimately decided upon by @johncoate, the world’s first-ever online community manager (at The Well, pre-Internet). It is accessible through a menu printed on top of every page. Have we let people know? How many people here are aware of that page? Not many, I’d wager.
In Edgeryders, this is not a big problem, because we model humans online as thinking adults. They own their words. They have a responsibility to be aware of what is considered excellent/good/acceptable/unacceptable behavior. The netiquette page is just there as a reference. We don’t care if people have read it or not, they are still held to the same standards.
Many people disagree with us. It seems to me that the Ix/Ux tradition, for example, was developed in a commercial context. Commercial companies model humans online rather as desire machines (thinking adults are tricky customers) and aim to gratify them. Though designers have amazing useful skills, I tend to be weary of the culture that spawned them. Paradoxically, this has come to influence my aesthetics: the abrasive, no-nonsense look of the Mother### website inspires trust in me, whereas I instinctively mistrust anything too sleek.
I guess what I am trying to say is “social media is broken, let’s be better!”. But even that would be naive, since technological affordances definitely influence social outcomes (though they do not determine them).