In the peer group, you do not need the set facebook variable.
Seeing as you sync that variable, it takes a moment after creation and the host then sets the facebook ID.
So you should remove the "set facebookId" from the peer group.
Also, you do not need to add the "sync variable peerID" in event 5. You can remove that.
The multiplayer object takes care of this. you only need to set them on host and peer end, as you have done.
Question: The facebook ID is all numbers, right ? remember that syncing object variables do not accept Letters.