Mastodon - A GNU Social-compatible microblogging server. Alternative to Twitter

Mastodon is a free, open-source social network server. A decentralized alternative to commercial platforms, it avoids the risks of a single company monopolizing your communication. Anyone can run Mastodon and participate in the social network seamlessly.

Statusnet - Microblogging platform powers Identi.ca

StatusNet is a microblogging platform. It helps people in a community, company, or group to exchange short (140 character) messages over the Web. Users can choose which people to “follow” and receive only their friends' or colleagues' status messages. It provides a similar service to sites like Twitter or Yammer.

pump.io - Social server with an ActivityStreams API

pump.io is a stream server that does most of what people really want from a social network. You post anything and my followers see it. It is an alternative to twitter. 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.

Elgg

A powerful open source social networking platform. It provides the components you need to build a fully-featured social environment, including Profiles, Groups, Activity Streams, Access controls, Blogging / Microblogging, Web Services API etc.

Apache ESME - Apache ESME (Enterprise Social Messaging Environment) is a secure and highly scalable

Apache ESME (Enterprise Social Messaging Environment) is a secure and highly scalable microsharing and micromessaging platform that allows people to discover and meet one another and get controlled access to other sources of information, all in a business process context.

Friendica - Facebook alternative

Friendica is a communications platform for integrated social communications utilising decentralised communications and linkage to several indie social projects as well as popular mainstream providers. It provides fully distributed communications protocols ("DFRN" and "Zot!") for securely sharing with your friends across the internet.

Twidere-Android

jsSocials - Social Network Sharing Plugin

jsSocials is a simple social network sharing jQuery plugin. It's flexible and easily extensible. Configure visual appearance. Choose one of several themes provided. Add any yet unsupported social network if needed. You can share via Email, Twitter, Facebook, Google-plus, LinkedIn, WhatsApp, StumbleUpon and lot more.

You Private Social Network

YourPrivateNet is for all people who are unsatisfied with how social networking giants, namely facebook, are dealing with privacy and the users data on their servers. This project gives users total control over their own personal data and who has access to it.

Hiitch (The Social Networking Platform)

Hiitch is a secure and advanced desktop social networking platform. It allows you to build a focused and private network of communities for your family, friends, company and etc. It gives you total control and freedom for your social networking needs.

VivoSocial

VivoSocial is a ASP.NET based social network platform built on top of the DotNetNuke framework.

peer-tweet - Decentralized feeds using BitTorrent's DHT

BitTorrent's DHT is probably one of the most resilient and censorship-resistant networks on the internet. PeerTweet uses this network to allow users to broadcast tweets to anyone who is listening. When you start PeerTweet, it generates a hash @33cwte8iwWn7uhtj9MKCs4q5Ax7B which is similar to your Twitter username (ex. @lmatteis). The difference is that you have entire control over what can be posted because only you own the private key associated with such address. Furthermore, thanks to the DHT, what you post cannot be stopped by any government or institution. Once you find other PeerTweet addresses you trust (and are not spam), you can follow them. This configures your client to store this user's tweets and broadcasts them to the DHT every once in a while to keep their feed alive. This cooperation of following accounts, allows for feeds to stay alive in the DHT network. The PeerTweet protocol also publishes your actions such as I just followed @919c.. or I just liked @9139.. and I just retweeted @5789... This allows the possibility for new users to find other addresses they can trust; if I trust the user @6749.. and they're following @9801.., then perhaps I can mark @9801.. as not spam. This idea of publicly tweeting about your actions also allows for powerful future crawling analysis of this social graph.

Voten - The code that powers voten.co

Voten.co is a real-time social bookmarking for the 21st century. It's real-time, beautiful, customizable yet simple.

Elgg - A social networking engine in PHP/MySQL

Elgg is an open source rapid development framework for socially aware web applications. Elgg is managed by the Elgg Foundation, a nonprofit organization that was founded to govern, protect, and promote the Elgg open source social network engine. The Foundation aims to provide a stable, commercially and individually independent organization that operates in the best interest of Elgg as an open source project.

Hotaru CMS - Plugin Powered PHP based CMS Engine

Hotaru CMS is a PHP platform for building your own websites. The most common use for Hotaru is social bookmarking. It has all the plugins you need to make your own Digg-style community. It is a framework for making websites customized to your needs.

social_stream - A framework for building distributed social network websites

Social Stream is an engine for Ruby on Rails. It provides a robust and flexible framework with social networking features and activity streams for building websites. There is a detailed guide in Social Stream’s website on how to get started, install Social Stream in your Rails application and customize it.

hubzilla - build community websites that can interact with one another

Hubzilla is a general purpose communication server integrated with a web publishing system and a decentralised permission system. If this sounds like a bunch of technical mumbo-jumbo to you, just think of it as an independent platform for sharing stuff online (publicly or privately). Hubzilla contains some social network bits, some cloud storage bits, some blog and forum bits, and some content management bits. These are all integrated within a common privacy framework - and it is all decentralised.

Voat - Media aggregator and an alternative to Reddit

Voat (pronounced voat as in goat) officially launched in April 2014 as a media aggregator website featuring user-posted content.

Vulcan - A toolkit to quickly build apps with React, GraphQL & Meteor

This is the Apollo/GraphQL version of Telescope, now known as Vulcan. It is a framework that gives you a set of tools for quickly building React-based web applications. Out of the box, it can handle data loading, automatically generate forms, handle email notifications, and much more. It provides simple building blocks such as posts, comments, forms, and modals, and makes it easy to quickly customize them to build modern, social web apps.

Steemit (Condenser) - The social application web front-end to the Steem Blockchain.

Condenser is the react.js web interface to the world's first and best blockchain-based social media platform, steemit.com. It uses STEEM, a blockchain powered by Graphene 2.0 technology to store JSON-based content for a plethora of web applications.