FacebookFacebook is going Open Source using a software package called Scribe was built using another open source tool developed by Facebook called Thrift.  According to Facebook, Thrift is its a software framework for scalable cross-language services development.

Scribe currently logs messages from PHP, python, C++, and Java code, and an ever growing list of possible languages from contributions of developers around the world.

Scribe, in itself,is not a social networking tool, but rather it is a tool for aggregating data from a large number of servers. That being said, having Facebook make it open source should allow for better Social Application to be created.