Diaspora

https://diasporafoundation.org/

Instead of everyone’s data being contained on huge central servers owned by a large organization, local servers (“pods”) can be set up anywhere in the world. You choose which pod to register with - perhaps your local pod - and seamlessly connect with the diaspora* community worldwide.

Gnu social

http://gnu.io/social/

GNU social is a continuation of the StatusNet project. It is social communication software for both public and private communications. It is widely supported and has a large userbase. It is already used by the Free Software Foundation, and Richard Stallman himself.

Pump.io

http://pump.io

Pump.io is a stream server that does most of what people really want from a social network. I post something and my followers see it. That's the rough idea behind the pump. You can post almost anything that can be represented with activity streams -- short or long text, bookmarks, images, video, audio, events, geo checkins. You can follow friends, create lists of people, and so on.

Known

http://withknown.com/

Known is an open source social network for you, your group, or your company.

Twister

http://twister.net.co/

Twister is an experimental microblogging peer-to-peer platform, implemented as a distributed file sharing system. Being completely decentralized means that no one is able to shut it down, as there is no single point to attack. The system uses end-to-end encryption to safeguard communications.[5] It is based on both BitTorrent and Bitcoin-like protocols[6] and is considered a (distributed) Twitter clone.

Friendica project

http://friendica.com/

Friendica

is our flagship platform.  It is an open source, free social web server running on the popular LAMP stack. Think WordPress or Drupal, but for social. It is a mature platform with over two years of development behind it, and provides a wide range of connectors to both legacy and indie social networks.

Friendica Red

Red is our next generation communications platform, currently under development. In building Friendica we learned a great many things about how to make reliable and scalable decentralised communications systems. We also know that to achieve critical success, we need to go far beyond "how to install servers" and provide a distributed platform which exists everywhere and can serve millions of people - while avoiding the traps of advertising monetisation (which brought us all the creepiness).