The facebook option is nice, but it is slightly clunky. I'm not sure how much interactivity you can get with facebook, but it would be incredible if you could see your online facebook friends and use medium to chat with them. Or your Skype or AIM friends for that matter.
Or perhaps an option to write on their wall instead of just sending messages.
very simple idea, at the moment if you close the sidebar, then medium closes, and stops running, it would be useful to have an option to run in the background so that your friends could still see what you are viewing, but the sidebar isnt taking up any precious screen room.
It might also be useful to have some sort of smaller notifcations in this mode, IM style maybe, or in the stausbar.