Slack’s problems are different from email but probably worse. We started using it at Uptake when there were only 4 people in the company, but as we grew I went through several phases in my relationship with Slack:

• 4–10 people: “This is great. I can set up channels for all the different activities we’re involved in and dip in and out as needed.”
• 10–20 people: “Hm, this channel setup isn’t working that great, let’s reorganize and set up some private rooms so everyone isn’t lost when they come onboard.
• 20–50 people: “Wow, this is getting noisy. Better pare down the channels I’m subscribed to considerably. Also, this digest email has to go.”
• 50–100 people: “This is turning into a monster, completely unmanageable. I wish people would email me things they actually expect me to read.”
• 100–150 people: “I have Slack installed but I never check it. It seems to be working great for other people, but if you need me you’d better email me.”
• 150–400 people: “My god what have we done. This is seriously draining overall productivity. How do we get rid of it now, or at least reign it in?”

Ultimately email is a much more mature platform, even though its hooks aren’t as clean or as nice. There are tons of third party tools for managing it, getting more info, etc. Gmail really is much more effective for me than Slack has been in a long, long time.