[1]: http://blog.stackoverflow.com/2010/04/do-trilogy-sites-need-...
I really like the "third place" idea, but I don't think the chat rooms have been successful in realizing it. Unfortunately, people just don't use them very much--even the StackOverflow rooms rarely have anybody to talk to.
I think the fundamental problem is that discussion in other parts of the site is asynchronous, like email, while the chat rooms aren't. The comments and posts stick around and allow people to talk about something across a longer period of time, which also allows more people to join the discussion. On the other hand, chat happens immediately--it only works for the people who are there at the moment and only works while everybody is on.
There is, of course, nothing wrong with chat in general. I just don't think it's the best match for the SE format.