I suspect that a lot of people have just internalised social media as a way to casually chat with their friends, without really grokking that their comments online are quite literally potentially visible to a large percentage of the entire species, forever -- including the people being discussed. That all those rules about rudeness apply to public-space interactions that you probably think of as 'private' because the target of the conversation isn't present in the conversation yet at that precise moment.
Like.. one way or another, your great-grandkids, their spouses, and their employers are all going to be able to read every single word you ever wrote on Twitter or in a HN comment or on your blog or nearly anywhere else, and all it'll take them is a random whim to look you up. A large percentage of everything you ever write/wrote online is effectively accessible forever, now, and available to anybody with an interest and an internet connection.
Most of the usenet posts I wrote as a early teenager 30 years ago (written under my real name) are still easily accessible online, for example, and it certainly never occurred to me at the time that that'd be the case. I wonder whether today's kids, who've grown up with all of these forever-archives already in place, are generally more front-of-mind aware about the situation and more careful with what they write than we were back in the early days before we really understood what the internet was going to become.