This new feature which is similar to the one on WhatsApp allow users to reply to a specific message in a chat, thereby making it easier for the recipient to know which message is being replied to.
This feature comes handy most especially in long detailed conversation so as to make easy in referencing a specific message you are replying to, and to also kill confusion that might arise in such threaded message.
To reply to a specific message on Messenger, the user will have to long press on the message where they will see a reply icon next to the reaction emojis. By tapping on it, users will be able to reply to that specific message.
Once the user has done that, the original message will display above along with the reply below. The message being replied to could be videos, emojis, text, photos, etc.
Earlier, Facebook also brought the feature that allow users to 'unsend' a message .... that is, Messenger users will be able to delete a message in a one-on-one or group chat. This feature too was existing on WhatsApp before Facebook decided to bring it on to the Messenger platform.
Facebook hasn't really seen an increase in its users. A report from AppAnnie in January shows that WhatsApp had more active users on Android and iOS more than the Facebook app. Also, a similar Pew survey also reported that Instagram was more popular among teenagers than Facebook was.
Right now, Facebook has in the work plans to merge all three social networking platforms; WhatsApp, Instagram and Messenger together.