Same here. We started using Slack because it was the only thing that could break the use of ad hoc AIM and Google Chat accounts.
I do think that the Slack homes for user groups and open source projects (the ones set up with auto-invite on Heroku) are direct replacements for "join us on Freenode" invitations.
which, TBH, a nightmare, because within slack you need to change "group" to be able to be present in multiple projects, or join multiple slack-irc-server...
