When you authorize Facebook for distribution, your music can then be used by Facebook / Instagram users when those users create content and share it on those platforms.
For example, your music could be used within:
- Instagram Stories
- Instagram Reels
- Facebook Stories
- User uploaded videos on Facebook that contain your songs (Similar to YouTube Content ID)
You, as an artist, could even use your own music in your own Instagram / Facebook Stories which has great marketing potential.