Original: Modern, Internet-wide, GUI-based messaging clients as they are known today, began to take off in the mid 1990s with PowWow, ICQ, and AOL Instant Messenger.
Simplified: Modern, Internet-wide, GUI-based messaging clients, as they are known today, began to take off in the mid 1990s with ICQ (1996) being the first, followed by AOL Instant Messenger (AOL Instant Messenger, 1997).