AIM was available as an app on iOS App Store's launch day on July 11, 2008. But push notifications for the iOS Platform didn't arrive until June 2009, so it was cumbersome -- daresay awful: messages only arrived when it was in the foreground [1].

Within a few months of push notifications first being available, WhatsApp was released for iOS, in November 2009.

[1] https://www.macworld.com/article/1135141/aim_review.html