The article is OK but misses a lot of major events that led users away from Yahoo Messenger. A big example is Yahoo's neglect of user ran public group chats. After a wave of media scares, Yahoo removed the ability to publicly list user ran group chats and neglected the group chat's feature set while other companies surpassed it. This led a lot of users to seek alternatives in the form of Skype, Mumble, Ventrilo, Teamspeak, Paltalk, Meebo, Windows Live Messenger and even AIM.
Another big factor was AIM's presence as a default app on Mac OS.
There's also AIM's proprietary communication protocol called OSCAR that was thoroughly reverse engineered which made AIM and its proprietary features more widely available on third party clients, leading to greater popularity than Yahoo among people who primarily utilized third party messaging clients.