why we haven't yet seen real social computing

Yes, people want to share discoveries and experiences with others, particularly with their friends, but not necessarily with their "friends" as defined by the social computing service du jour, and, in most cases, emphatically not with that service interjecting itself into the relationship.

A real social computing system would be more ubiquitous than the telephone network, and easier to use than the postal network. It would, at least in principle, include everyone on the planet in one way or another, even those living outside the reach of ground-based communications networks and on the economic fringe, unable to afford a phone much less a computer and a satellite datalink. It would be all about allowing people to connect with the other people with whom they wanted to connect, individually, in groups, and in context, as well as to avoid the whole range of threats and parasites. And, as much as possible, it would get out of the way and allow those connections to play out as naturally as possible.

The closest thing we have to this at the moment is internet mail, which, rather than being a proprietary service, relies upon the interoperability of thousands of services, based on a collection of standard protocols. For all of its inadequacies, email is the best available model.

That's not to say email should or even could serve as the basis for that social computing environment of the future, which is likely to require a fresh start. But as a standards-based experiment in interoperability, it can serve as a starting point for thinking about what might be required.