iMessage was introduced in 2011 alongside iOS 5. It was believed to be Apple’s answer to platform-exclusive messaging platforms like BlackBerry’s BBM and Samsung’s ChatON at that time. Prior to that, the Messages app was mainly used for sending/receiving regular text messages and MMS. However it seems that thanks to more iPhones being adopted on an annual basis, along with more iPads being sold, it looks like Apple’s iMessages is quite a hit amongst iOS users.