It does not use the MMS protocol when you send a group message via Facebook Messenger, WhatsApp, Viber, etc. Instead, it sends a group text message, and replies are sent individually to each person who receives the message. If someone replies to the group message, those replies are sent to everyone who received the original message, and they return as individual texts. This happens because the reply is sent as a text message, not multimedia. So, if you want to see responses to your group message in one place, choose "Group MMS" over "Group SMS" as your messaging method.