Each IM contains the first few hundred characters of the post, along with a small permalink that takes you straight to it. Like the recently introduced live feed, the idea is that you can quickly track and respond to the activity of your friends while they're still around.
To help turn down the fire-hose of information, users can whittle down whether they want to see everyone's interactions, or just those from friends. This includes the option to track just newly posted items from any feed, or any comments posted to items they've submitted.
While this is useful for desktop users, it's also a nice way to bring the FriendFeed experience to your phone. If you've got a generous text-messaging plan you can reroute replies or new posts to very specific rooms to your phone if you know your carrier-specific e-mail address. Short of a phone application for the likes of the iPhone or Android phone, this is the easiest way to keep an eye on things without trying.