Software in the 1980s, when usability was "invented," was all about computer-human interaction. A lot of software still is. But the Internet brings us a new kind of software: software that's about human-human interaction.
Discussion groups. Social networking. Online classifieds. Oh, and, uh, email. It's all software that mediates between people, not between the human and the computer.
When you're writing software that mediates between people, after you get the usability right, you have to get the social interface right. And the social interface is more important. The best UI in the world won't save software with an awkward social interface.
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.
We all learn about the importance of making are softwares more usable. All the usual talks of usability. Joel Solsky writes about "the next level of software design issues, after you've got the UI right: designing the social interface."