BuddyPress.org. Just stumbled upon it again when it came to self-hosted social networks. BuddyPress somehow always flies under the radar, but it's quite an interesting development with all the features of WordPress - including, for example, all login integrations and similar. And there are also separate plugins for BuddyPress, such as the BuddyPress Media Plugin, with which you can create your own Flickr from a BuddyPress installation. And there are countless more things, up to commercial plugins for various purposes. And additionally, you can use all WordPress themes and build on them.