r/sysadmin primarily but it's not that great.
Unfortunately a lot of what used to make this area great has slowly died... IRC being the loss felt most keenly. It still "alive" and a few folks like my self still hang out in various channels but it's nothing like it was when I was getting started. Mailing lists being the other one, certain dev lists and network groups are still really active and frequented by folks that run the big NOCs but also in a sharp decline.
Some of it's being replaced by random Slack instances but this is shit.
Unsure what can be done other than try bootstrap a community.