Messages is in the bar at the top, and a Message button sits on the page of anyone you are friends with. A conversation is between two people; there are no group threads.
Who you can write to
People whose friend request you have accepted, or who have accepted yours. Not strangers, and not somebody with a request still pending. It is the same list as your Friends page.
If you stop being friends, the conversation stays where it is and neither of you can add to it. Nothing is deleted, and connecting again picks it back up. Losing a conversation you were part of is a bigger thing than ending a connection, so ending a connection does not do it.
Nobody is told when you read anything
This is the same decision as having no like counts. A read receipt tells somebody you saw their message and did not answer, and it tells them what time you were awake — which is exactly the watching this place says it does not do. The cost is real: you cannot tell whether a message landed. We think that is the better trade.
Pictures
Up to four per message, and pictures only — JPEG, PNG, WebP or GIF, up to 10 MB each, shrunk in your browser first like anywhere else on the site. No video: private video is where storage grows fastest, and a large file in a conversation nobody revisits is the worst version of that.
A picture in a message is stored the same way as one in a post: the bucket is private and every link is signed and short-lived, so it cannot be reached by guessing the address. What it is not is encrypted — see below.
Taking one back
Delete on your own message removes the words and any pictures, and leaves Message deleted where it was. The mark stays on purpose: quietly removing a message somebody has already read rewrites a conversation they remember having.
Stopping someone
Blocking ends it. A blocked person cannot write to you, and the conversation leaves both inboxes. Unblocking restores it unchanged.
What this is not
For anything where the difference matters, use a tool built for it, like Signal. Being told plainly is worth more than a reassuring sentence.
There is also no live delivery: the page checks for new messages about once a minute rather than the instant they are sent, so a reply can take a moment to appear. Reloading fetches it immediately.