Facebook Messenger may soon allow users to sync their Instagram accounts1
Facebook Messenger already allows you to add your Instagram account, but there's no option to sync it at the moment. The new feature is supposed to replace the basic Instagram Account option, which can be found in the Profile, under the People menu, along with Sync contacts and Blocked people options.
Once you sync your Instagram account with Facebook Messenger, it will give access to your Instagram username and account to your friends on Messenger.
The new feature may already be available to a limited number of users who own both Messenger and Instagram accounts, which suggests Facebook is actively testing the sync option to see how it actually works in real life.
It remains to be seen when exactly it will be enabled for everyone, but since it's not really a major feature it's unlikely there will be any official announcements unless it goes live at the same time with other, more important improvements.