Every family in the world uses IM, and Slack isn't very different from any of the hundred other options out there for group messaging. So not too surprising that families are using it.
Similarly all the other software mentioned can be used for simple calendaring, TODO lists, reminders etc., all of which most people here undoubtedly use in some form to manage their life. It's just the association of these brands with "work" that has people going WTF.