Mission*Social is (from their website): "a unique workspace for social enterprises in San Francisco, providing affordable office space, shared resources, and the opportunity for informal dialog and collaboration among the organizations.
The space was originally home to the non-profit emerging market IT provider Inveneo. They decided to expand the space and open it up to other socially oriented firms. According to a CNET article, 8 organizations now call Mission*Social home.
As a post on the Cool Town Studios blog points out, socially oriented coworking spaces have been around for some time. The Hub, for example, was founded in 1995.
As a board member of a non-profit, social enterprise, the Society for New Communications Research, I can attest to the need for inexpensive, shared office space and the collaboration opportunities they provide.