I like the AskHistorians subreddit, but I haven't found much else. I think AskHistorians and Stack Exchange have a somewhat high "barrier to entry" for actually answering questions, which might be part of it. (I mean this in the sense that you're going to get down-voted to hell if you don't really know what you're doing on Stack Exchange, and I think you actually have to be a historian to answer on AskHistorians.)

HN is more of a bad habit for me than anything else, unfortunately. I end up lurking comment sections just to disagree with everyone and turn it off.