Summary: Web and online communication and interaction is asynchronous. As such the delay, be it a matter of seconds or several days, disintegrates the sense of "being there" and "shared time" that characterizes co-presence, simultaneous, and spontaneous interaction. People have the time to consider themselves consciously, to tell rather than talk, and some of this comes out in online and social media as cold, distant, and self-oriented rather than other-oriented. And yet a great deal of talk now happens in a mediated fashion. The implications for the design of social software, online interaction, online community, and social media are significant. And they will become very interesting as these media become more synchronous.
Transitivity describes our availability to communication through technology, and in particular, the network. Phone calls are for the most part point to point. (Though we could, who makes social conference calls?) Email and other networked technologies, however, excel at "group" interactions. Group addressing and cc'ing are certainly some explanation for this. It's cheap and effective. And quoting somebody in conversation is a simple matter of forwarding-no introduction required.
Transitivity in network terms describes a node through which communication travels. It obtains a curious twist when we use it to characterize social behavior. For it now suggests that there's a way in which we're all nodes in a network of communication, and whose flow we either pass along or not. The concept's an interesting one, because in asynchronous media, social phenomena depend upon transitivity of groups and individuals to the communication flow. Viral and word of mouth advertising depend on this effect, and rather than spam everyone with messages rely on individuals to pass them to friends instead. (The difference is extreme, but that's another discussion.)