Open Source Social Networking App. – Diaspora
It includes most of the features to be expected like friendships, status messages, photos, etc.
The application's most original part is the way the user-data and so the privacy is handled: all personal data is stored wherever you want like on an hosting account or on your personal computer.
And, considering it is open source, users will be able to attach their profiles (including all personal data) to a network built with the system.
Diaspora is built with Ruby and still in the early development phase. However, seems like we'll be hearing more about it in the near future.