When there's more time, I'd like to think a bit about axes on which to organise social software.
There's telic [goal directed] vs atelic. Bugzilla is definitely goal directed -- but how about some of the features of Outlook?
There's certainly a continuum from pairwise to group. As much as email is used for groups of people (mailing list, and so on), I think its applications still come from a pairwise mindset: adding people to a To or Cc list is just adding people to a list. The group doesn't take on its own identity. And it's always easier to lose people from the group than gain them: people can't loiter nearby and wait to be invited.
Even listservs are often pairwise: they're often broadcast. But these things bleed into each other.
The point of mediation of the communication makes a big difference. Is it one where you choose who to talk to - like email, again - or is it one where you choose to whom you listen? The latter is like the www, where your experience is determined by you the listener (the reader) rather than the speaker (the author).
Also, how much time-binding is present? That is, how much is your use of the medium changed by previous uses of the medium? In electronic communication, the medium usually stays the same, or changes very slowly. In the real world, old messages accrete in the environment.
Formality vs informality.
Active vs passive, or even, deliberate vs accidental. More about that under communication theory.
There are more than just axes that would be good to explore. Some questions: