I don't really remember but when it appeared there was an option button/link and I followed it and one option is to switch off the sidebar. Never use messaging really.
Looking just now I have the old style tab, if I click it the sidebar pops open, there's a "cog" for settings in the bottom right corner. IIRC you have to set yourself as "unavailable" to all and then use the cog to select "hide sidebar". Try that.
Hm, I think we had a slight disconnect in our topic!
In the last week or two, Facebook changed their messaging UI so that you cannot get a list of all your buddies that are online to chat (it guesses who you would care to see) among other mostly cosmetic changes. AFAIK there is no way to use the chat UI that was available a month ago now (or see if a particular person is online unless you specifically type in their name).
