Humhub community edition is a free, self-hosted, open source, social network application. You can create a social network (or social intra-network) for your club, community, organization, or whatever. You can get a feel for it with a hosted, 3-user/500MB account at the Humhub site — this is the free tier of their hosted enterprise edition.
A colleague and I are evaluating Humhub as an alternative to more fully fledged CMS/Groupware products for a small team (a dozen members) working on community development projects. I’ve installed Humhub (version 1.2.5) community edition on a local SBC (NanoPi M3) using ngrok for evaluation access. The installation/setup process is fairly straightforward (following the documentation at the humhub site), and I used php-7.0, nginx, and mariadb without any hitches.
First impressions are very promising. The interface is clean and simple and Humhub provides the basic posts, comments, and private messaging you might expect. It also provides for file sharing as well as Spaces (projects, groups, special topics or events) with their own posts/streams and access rights. These Spaces can be enhanced with modules for wikis, custom pages, calendars, tasks, and polls. This brings it beyond a simple private social networking/communication tool and makes it a viable option as a collaborative community environment.