ICQ was once the king of the hill, then Msn messenger, for a short time google talk, then WhatsApp. People find alternatives, things change all the time
But every time there are more people than the previous time. This means it's getting harder for everyone to switch.
And then there's this: I have no idea about other countries, but over here in Russia, instant messaging got somehow consolidated over time. I used to have to run ICQ (as qip), Mail.Ru agent, Skype, and VKontakte (in a web browser) to be able to talk to everyone. Now Telegram and VKontakte cover 99% of my IM use. Some people do use WhatsApp, but not in my circles, thankfully. Some people have managed to delete their VK accounts without inconveniencing themselves and others much, but I'm not able to do that. Overall, the more users a social app has, the harder it becomes to switch away from it. Also — those old IM services only had one-to-one chats, but all the current ones have group chats, which make switching immensely harder. Can't imagine myself switching away from Telegram in the future for example.
