Joel Spolsky writes a great piece titled—It's Not Just Usability. The article is about 2,500+ words—an interesting read, replete with Joel's opinion of the current brood of Social Networking Services.

A couple of great wrap-up paragraphs at the end of this article:

Social interface design is still a field in its infancy. I'm not aware of any books on the subject; there are only a few people working in the research side of the field, and there's no organized science of social interface design. In the early days of usability design, software companies recruited ergonomics experts and human factors experts to help design usable products. Ergonomics experts knew a lot about the right height for a desk, but they didn't know how to design GUIs for file systems, so a new field arose. Eventually the new discipline of user interface design came into its own, and figured out the concepts like consistency, affordability, feedback, etc., which became the cornerstone of the science of UI design.

Over the next decade, I expect that software companies will hire people trained as anthropologists and ethnographers to work on social interface design. Instead of building usability labs, they'll go out into the field and write ethnographies. And hopefully, we'll figure out the new principles of social interface design. It's going to be fascinating… as fun as user interface design was in the 1980s… so stay tuned.

Some of my best social software cohort friends are ethnographers and sociologists—avid field workers, prolific bloggers, and excellent communicators, one and all.

BTW, Joel Spolsky has a discussion forum set up on Social Interface Design over at Joel On Software.