When people post something to their wall or profile, all of their friends see it on their walls. A message can be sent by clicking on a friend's name in one's friends list, which takes them to the friend's page, and clicking on the link in that page for sending that person a message. I've only ever had chats with one person at a time, so I don't know how that is done differently than a group chat.

Like I said... ah well.