Just a couple points about Facebook Messenger you should be aware of
1. Chats are not monitored
2. Secret encrypted conversations are possible. The dangers obviously with this is parents are kept out of loop and messages your teen thought were private can be used against them if the other user takes a screen shot of conversation.
3. You can locate friends on a live map (make sure you know who your child is accepting as friends)
4. In app purchases can be made and you can send or request money transfers.