Friendica is a federated social network. You can install it on your own network. It is opensource. It is not centralised like email you can choose the provider but you still can communicate with differents servers.
The Friendica Project is a world-wide consortium of software developers creating decentralised social platforms and technology for the coming post-Facebook world. Friendica is not as flashy and well known as some of the other projects working on a decentralised/federated social web.
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. Your « friends » can be from Facebook, Diaspora, Twitter, Identi.ca, weblogs and RSS feeds – and even email. No other project has ever brought together the social universe into a single federated stream to this degree. We have pioneered ground-breaking technology such as decentralised photo albums with working privacy controls and cross-site wall-to-wall posting via hidden single sign-on. Not even Facebook will protect your private photos from « fuskers » (look it up, but be warned – it’s real creepy). If you make a Friendica photo private, it’s private.