I still think Slack is the greatest thing since sliced bread, but I agree with you. If I need to send something non-time-sensitive to a specific person I will

I still think Slack is the greatest thing since sliced bread, but I agree with you. If I need to send something non-time-sensitive to a specific person I will still go to email or another channel (1:1 meeting, Basecamp message, Trello card, ticket, etc) rather than sending an @mention. If it's not specific to an individual, then I think dropping random things in channels works reasonably well asynchronously as it lets everyone decide when they have time to read the given channel, and of course it shows up in search for later reference. Slack stretches the boundaries of real-time communication quite a bit through clever design choices, but that's still its fundamental nature.