Appolgies so I guess I didn't find this post for some reason, even though I searched...
But anyways, vBulletin recent update has a IM within their PM system. If both are online they can engage in real time messaging. It works like a normal PM because it's stored all the same. So you could always go back. When you start a PM/IM all messages are stored in the conversation because each PM/IM had a topic title as well. The administrator can still limit the user group for how much storage they want. But again, if 2 users were engaging it's a seemles conversation.
I should point out that other's have said IM would drive away the user base? That logic is invalid.
It would not drive away the users because they are limited to just whomever they are talking with, the admin determines how many people can be involved in one PM/IM, so threads would not be come irrelevant. In fact, threads someday itself could become instant as well... It's all about Users Experience and how we can catch up on modern internet technology.
Facebook still has Wall Posts and Messanger. Do people use messenger a lot more then wall posts? Probably not. So in this case people won't be hermits on your site. Messanger, just like Facebook's messenger is used all the same.
I think people's fear of social media taking over is whats killing communities. UX plays a very important role.