What are you talking about? What functionality does an instant message application need that isn't supported? It can get push notifications if someone sends you a message. You could even leave your app running for up to 10 minutes in the background waiting for updates if you wanted (which probably matches actual usage pretty well).

What you can't do is have your app sitting in the wait queue for data coming in over the wifi because that would mean the wifi card has to run constantly. Ask Android how quickly doing that can drain your battery.