RingSide Networks is an open source offering that enables any website with social networking capabilities by providing implementation for both Facebook and OpenSocial specifications. RingSide acts as a powerful container hosting social information and as a bridge to Facebook social information platform hence providing interoperability. The only issue with this is its still in Beta.
Liferay is also an open source offering which provides both enterprise portal and social collaboration platform software. The solutions provide components for Web 2.0 features like Blogs, Wiki, Calendar, etc. It also provides a way to host social graph and promises OpenSocial Apache Shindig integration.