I just realized that arguably I started working on the social web in ~1999, not ~2005.

At Aviatis, I created this feature where our users -- who were engineers -- could share semantic objects of certain engineering diagrams shown in a browser with each other with deep context for purposes of collaboration around those objects. That's about as social as a use case as I can think of...

That was Java applets rather than HTML, but I don't think the users cared (except for the load times ...)