In social media generally I think DM's are toxic because inevitably you get spam-scams the moment you poke your head up.

At work I use DMs on Slack when I expect a response from somebody specific ("here's the report on Y, and here is what we should do" or "WTF is the source code for that foundation package in our Nexus?") or if I know there are people on the channel who will be offended by irrelevant chatter. Otherwise I use the public channel.