While you’re in a conversation, you’ll see reactions to messages in the lower corner, where you get a summary of how many people reacted. Tapping on that will tell you exactly who reacted how. And when someone reacts to one of your messages, you’ll get a small animation. If you don’t have Messenger open, you’ll get a notification telling you that someone has reacted to a message of yours.
Reactions will be available for all messages – text, stickers, videos, images, GIFs, and even other emojis. And all of this works in all conversations, be they are one on one or of the group type.
Mentions on the other hand are handy for group chats when you want to get someone’s attention. Just type the “@” symbol followed by the first few letters of that person’s name or nickname. You’ll get a list of people that you can pick from. Then write what you want and that participant in your chat will be notified that you’ve mentioned him or her.
This will be achieved through a new type of notification, which lets them know that you called them out specifically.