From other instances, or if you view Lemmy from your browser while signed out, people's usernames are shown in full. If you share a link to this post with yourself and view it while signed out, you'll see your full username - SouthEndSunset@lemm.ee

The part after the @ is your instance, and just for completion, your username is the part before it, making you SouthEndSunset on the Lemm.ee instance :)