* Look into Diaspora. (https://diasporafoundation.org/). Upside: It's basically a self-hosted facebook. Really cool project. Downside: Unlike facebook, there's no fake/pushed content so it tended to feel stale.

* Look into hosting a forum (e.g. phpBB). Forums are excellent because they don't lose old information like facebook does. When someone says "Hey what's the policy on dogs?" three years later I can search "dogs" and find the answer. Downside: They're not pretty, not full of pictures and no infinite scrollingz. sadge alfababies. Kidding aside, if you do try a forum, be sure to not offer a bunch of niche subtopics. The more subtopics the more stale the forum feels overall. Just stick to one main topic until someone asks for a second.

* IRC chat. I hosted an IRC group for several years at work and it worked great. We only killed it when we decided to move to an enterprise communication app.