GitHub is a social network - with far better UI/Ux than all the other social networks other open source software tries to use. It is also the location from which many packages do their releases and a very helpful tool when managing large scale projects (usually). GitHub also provides a self-hosted version, but imo they are slow with feature delivery in some cases and an open source project that really dug in would be great.

I know about gitlab et al, but nothing really quite creeps up to the same level of GitHub