Two ticks
Direct messages landed this morning. The first thing anybody asked for was the thing every messenger has and nobody thinks about until it is missing: did it arrive, and did they read it.
So your own messages now carry a mark next to the time.
One grey tick means the message is stored here. You pressed send, the server took it, nothing else is known yet.
Two grey ticks mean their client has pulled it. Their browser or phone has the message sitting in front of them, whether or not they have looked.
Two blue ticks mean they opened the conversation past that message.
The mark only ever appears on lines you wrote. A receipt on somebody else's message would be telling you what you already know, so there isn't one. The same mark shows up in the conversation list, on the newest line of each thread, when that line is yours.
What the middle tick actually measures
Here is the part worth being precise about. Messages travel over plain requests right now, not a live connection, which means the other side's app asks us every half minute whether anything new has arrived. That question is what turns the second tick on. So "delivered" can lag by up to thirty seconds behind reality, and on a device that is fully asleep it will not turn on at all until the app wakes up and asks.
That is the honest limit of what we can see, and it is the same limit every polling messenger has. A live connection would tighten it, and that work is already written down as the next phase for messages. When it lands, the ticks get faster without changing what they mean.
No hiding them yet
There is no switch to turn read receipts off. Plenty of people want one, and it is a fair thing to want: a blue tick tells somebody you saw their message and chose not to answer yet. It was left out of this first version because a half-built privacy switch is worse than none, not because the idea was dismissed. If you want it, say so and it goes in.
While we were in there: the unread badge on the phone now counts private messages too. Before today it counted only chat rooms, so a message addressed to you personally could arrive behind an icon showing nothing at all.