When you send out a GROUP MESSAGE - EVERY SINGLE TIME someone replies - you get a notification on your screen and your "inbox" blows up.
When a person "leaves the group" that means they have LEFT THE CONVERSATION - they walked out the door. They didn't change their settings, they didn't turn their computer off, they didn't close facebook, they LEFT THE CONVERSATION.
There is a difference between having a "group" and sending a "group message".
A group: can post on a wall, ask questions, etc. you can either add or remove people depending upon your status - admin or not. you cannot "hide" comments in a group, they are there for all to see. If you do NOT want someone to see it - you have to block them or remove them from the group.
A group message is just like an e-mail - once the initial information has been disseminated, people leave because after that? It's all down hill from there. When in a group message, you will see that Janet left the conversation.
So what does it mean? It means they were done with the conversation, got what they needed and left.