There have been three major changes to 21st century writing: (1) writing is more informal, or “looser”, as @wynkenhimself puts it; (2) writing is more voice-driven, more personal (you can get a sense of what the people above are like by reading their tweets and Facebook posts, and (3) writing is more audience-specific. The tweets and Facebook replies above were composed as part of a conversation with a person or specific group of people (me, or me and all my and their twitter and Facebook followers). All were written to me particularly (and they knew when they wrote them that I am a professor of writing and a writer interested in new technologies. Their responses may have been different if the question was asked, say, by their children). And, as @jbj and @wynkenhimself show, sometimes one reply to me leads to a new conversation between two other people.
It can be hard to know how to engage in this type of writing. You might feel a bit lost and unsure of the tropes of twitter, say. But chances are, you are more comfortable with writing than you were 10 years ago. Why? Because you do it more. Think about it. Today, you may text, email and Facebook dozens of times a day. In the 20th century, you may have gone weeks or months without ever writing anything (though you probably talked on the phone more than you do now).