I've been thinking about these ideas too, albeit in a slightly different way. Besides context, which other commenters have already covered, one missing element from ff, twitter and facebook is finding related information (similar posts, subjects, etc.) although there are some tools like tag clouds that do this in a rudimentary way. I think the next generation of these tools will not only control the time element of the conversation but have more "data mining" and subject mining tools to help discovery of information.