Lobste.rs for technical stuff. But most security related conversations by security SMEs aren't happening online anymore. We have specific user conferences and regional user groups now.
The cybersecurity industry is almost entirely located in the Bay, Seattle, Tel Aviv, and Blr/Hyd, so the really active user groups are mostly in those cities.
Cybersecurity goes hand-in-hand with IT, DBA, Networking, DevOps, and OS/Systems Programming - all functions that were previously looked down upon over the last 15-20 years.
Furthermore, most American CS programs made OS internals, Computer Architecture, or Distributed Systems optional, so the junior portion of the ecosystem doesn't exist in the US anymore.
I don't use Lobste.rs anymore because the owner irrationally blocked the browser I'm using, and I refuse to switch to a different browser just to read Lobste.rs. The owner seems like he has some issues to say the least.
