I think there are in fact only two attributes — Groups and Conversations — which are on the ‘necessary and sufficient’ list (though I have expanded the latter to Conversations or Shared Awareness, for reasons described below.) I doubt there are other elements as fundamental as these two, or, put another way, software that supports these two elements is social, even if it supports none of the others. (Wikis actually come quite close to this theoretical minimum, for reasons also discussed below.)