Twitter is effectively a CLI. Posts that start with "D" form a direct message rather than normal status update. You can also issue other commands which follow people, edit your profile, and more.
It's a CLI with terrible feedback. So many people (including people whose jobs are in some way related to Twitter) don't know that when you start a message with an @reply, only your followers who also follow that other person will see it.
A few years back, when more people used the SMS interface to post, it was common to see people occasionally slip and post what was supposed to be a direct message. I don't see that anymore now that people use GUIs.
There was a lovely gaffe last year when @neilhimself accidentally did a public tweet of his hotel phone number in what was supposed to be a DM to his wife.
Also, I did not know that about the @reply. Twitter needs to come with a --help.
A few years back, when more people used the SMS interface to post, it was common to see people occasionally slip and post what was supposed to be a direct message. I don't see that anymore now that people use GUIs.