The majority of people I talk to on Discord went the route of AIM/MSN/Yahoo -> Skype -> Discord. The big thing that I found was that the concept of Discord server was so significantly better then using group messages on anything before it that everyone immediately flocked to it. Just the concept of channels is a new one to a lot of these people. The majority were not gamers (never used Teamspeak/Vent/Mumble), or particularly technical people (IRC would not be a good fit).

So props to Discord for making a chat client that's as easy to use as Skype was.