Today, iMessage is probably the best single messaging platform, which is pitiful because it has key design and UX flaws.
I think in the case of AIM, it existed by virtue of and thrived under corporate neglect. The others withered as the complimentary services withered (Yahoo) or the corporate strategies shifted in inscrutable ways (Microsoft, Google). It was a weird scenario where MS and Goog behaved similarly... Microsoft with their mysterious purchase and subsequent neglect of Skype and Google with their drunken monkey product management process.