Company wide/support channels, your basically at liberty to completely ignore these. Some people crack the odd joke in here, camaraderie in a team is nice, but don't feel its compulsory. I mute them, though they're are great places to be able to search!
The channel for your immediate team/specific projects, your not obliged to monitor this but it'd probably be helpful for the team if you checked in twice a day or so, you might be able to guide or unblock a teammate with knowledge they don't know you have.
And direct messages. Again, its up to your to triage these, if its unimportant just mark unread and come back later.
The important point of slack is that communication is open, which brings network effects, I cant count the number of times I've stumbled on information that's been directly helpful to me. However, just like any other open information portal, most of it isn't directed at you, so don't worry about it.