I've noticed that the chat rooms aren't too busy.
I wonder if part of this is that users cannot find them?
Once you have a reputation of 20 or more you will have the Talk In Chat privilege.
To find the GIS Chat Rooms since the New top bar is live:
There's also a link to chat at the bottom left of the page, under "help".
Note: The description below applies to some GUI experimentation that was happening in 2011.
There's also a link to chat waaaaay down at the bottom of the page, between "blog" and "data".
And you might occasionally see links in the right-hand sidebar while reading a question.
The one at the top comes and goes - it'll be there if there hasn't been a recent blog post (but when the blog is active, it'll be a link to that instead).