We use a topic Random Thoughts for things like that. So far that works well for us.
Personal Messages are used like a “private chat”. We use it on my forum and it’s being used on another forum I’m on, but as a “private ‘group’ message” with about 8 members. It would be interesting to see how this would play out in a Group Chat.
As @sam points out in the chat message, one really nice feature is being able to see both the Chat box and other topics at the same time. Even though I noticed it, I hadn’t given that a thought. A 
for that!