Last day I came across an article explaining how to send an encrypted memo here on steemit. You just have to put a '#' at the very beginning of your memo. It seemed to work for everyone except me. I got an error. It turned out that it was because I was logged with my posting key only instead of my master/owner key.
I was very surprised because this meant that everybody was using their master/owner key to log in maybe without even knowing it.