Personally, IRC is a lot more about reaching people you don't know than those you do. Though I do use it to communicate with groups of friends as well.
Lately IM has been catching up to this, but them all being incompatible walled gardens means you can't build anything that survives the company failing (AIM is IIRC being shut down) or being bought and merged into some other network (like skype). Matrix is a good try for an alternative, as is XMPP. Any true successor to IRC will need to be federated between independent servers, completely open and as close to the simplicity of the protocol as any new features allow.
Discord, Slack, Flowdock, Skype, Messenger, WhatsApp, Telegram (AFAIK) and any other closed server single-provider walled gardens can never replace IRC for all use cases.