Database design for internal messages (e.g. facebook message)

In the following database design, how can I mark an unread message, do we know who reads the message from either the sender or the receiver?

(i.e. when person X sends a message to person Y, the message will be marked as unread for person Y until person Y reads this message, but it was marked as read for person X because he sent the message).

Message

• Id (PK)
• Subject
• Content
• MessageTypeId (FK) - message, update (public board), notifications, etc.
• UserId (FK) - creator
• Createdate
• ReadDate

MESSAGE_COMMENT

• Id (PK)
• MessageId (FK)
• Content
• UserId (FK) - creator
• Createddate

USER

• Firstname
• Lastname
• Username
• Password
• Isactive
• etc ... etc ...

MESSAGE_TYPE

• Id
• The code
• Name

EDIT: The design seems to be incomplete.