From the tile application for messaging if you have added Facebook to your account those contacts will show on your contacts list. The messenger app will tell you whether the recipient is "on Line". If they are not on line you will get a message that the
recipient is not on line but you can still message this person.
If the Facebook contact is "on-line" there will be an exact duplication of the message you send to this person appearing on the chat display about 1.5 seconds after the first one appears on screen. The client on the other end only sees one message the duplicate
is not sent.
If the contact is not "on-line" on Facebook there is no duplication on the (my) screen. The message does go to the recipient and it is there when they log on to Facebook.
It's like Facebook servers are mirroring or reflecting the message back to MS when the client is available for chat