I wasn't trying to offend. In my experience, the kids who didn't read enough tended to think proper spelling, capitalization and punctuation weren't important, even in the days before texting and IMs. As they grew up and began to use the English language in a less superficial fashion, they started to see the importance of writing properly.
I've seen multiple friends go through this phase, at different points in their lives. One (a disabled guy who doesn't work -- he's a great friend though, I hate calling him out), still refuses to spell, capitalize, or punctuate. Everything he writes is just an unformatted list of misspelled words. Not coincidentally, this is the only friend who doesn't read for work or pleasure. All he reads is facebook statuses and the weather channel.
Maybe I'm being uncharitable, but in my experience, people start out thinking punctuation and spelling are matters of formality, eschewing it as the concern of old folks and authority figures, but eventually come to realize it is about readability. But maybe there's a social signalling that I'm unfamiliar with:
this note is short and readability isnt a concern as much as signalling im not a stuffy outsider
While this note (and the rest of this post) is something I want you to consider, and therefore the utmost importance is legibility, readability and clarity.