In this post, AdPulp does a nice job of breaking down some of the differences between monologue-driven communication, like e-mail and blogs, versus dialogue-driven communication, like IM and Twitter:
People want to engage by creating stories with others, as they would in real life. When one tells a story in real life, other people add to it in real time (which can be annoying to the storyteller). Twitter mirrors this, whereas a blog is more traditional in its storytelling structure. On a blog a writer offers something, then comments come in, but it's not a conversation just like email is not a conversation. IM and Twitter are conversations; thus, the excitement around them.This may help explain why IM is gaining preference over e-mail. And while I still don't entirely understand the Twitter phenomenon, the analogy to a conservation seems apt.