Looking back over history my first Messenger was AOL's AIM and then quickly I was using ICQ. Yahoo Messenger came along about the same time as Windows Messenger which actually was able to access the SIP stack buried inside Windows 98. It was back then that I made my first VoIP call via Webley's never released VoIP platform.
Skype pretty much wiped out all the IM clients but today other are really taking their place. Mobile apps like WhatsApp and a few others rule the roost in mobile, the reduction in cost for SMS and services like GoogleVoice have also taken the need for IM down a peg, but with services like Slack and HipChat taking a bigger piece of the desktop and knowledge worker messaging activity, greater interoperability with other apps and hooks into them via API access from IFTTT and Zapier even Skype's days may be numbered.