In Wired, Clive Thompson describes the "telepathic awareness of the people" that makes Twitter so engaging. I described this as "ambient friend awareness" a while back, but I love Clive's analogy: "It's like proprioception, your body's ability to know where your limbs are.... Twitter and other constant-contact media create social proprioception. They give a group of people a sense of itself, making possible weird, fascinating feats of coordination."