A while ago, I had a chance to review an app called Beluga. At first, it looked like it could be a cross-platform answer to BBM, the popular instant message service that comes standard with all Blackberrys. As a cross-platform solution, it worked well.
However, one of the problems with Beluga was that even though it was good in theory, there wasn't the user base to keep it afloat. You either had to convince your friends to use an untested product, or go back to SMS.