When the WebKit window isn't in focus it will lose the connection to Facebook chat. I've tried other browsers and they retain their connection. It was only affected after a build I downloaded a week ago.
I too have recently started to have this problem. I find that even when your browsing facebook, the chat disconnects after about 1 minute.
I can confirm I see this behaviour also.
I'm sure that Facebook chat hasn't remained broken since 2008.