Mark latest Chat message as read by count of users or other user you are chatting with
In IM, show count of users who have read your message. If 1:1 chat, that the person on the other end has "read" your message. If multi-party chat, show count of users that have read the latest message.
Some have developed the habit of "clicking like" on messages to show they have read it. This can replaced by an automatic counter showing count of recipients that have read the item.