In essence, Facebook users didn’t think they wanted constant, up-to-the-minute updates on what other people are doing. Yet when they experienced this sort of omnipresent knowledge, they found it intriguing and addictive. Why?

Social scientists have a name for this sort of incessant online contact. They call it “ambient awareness.” It is, they say, very much like being physically near someone and picking up on his mood through the little things he does — body language, sighs, stray comments — out of the corner of your eye. Facebook is no longer alone in offering this sort of interaction online. In the last year, there has been a boom in tools for “microblogging”: posting frequent tiny updates on what you’re doing.

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Psychologists and sociologists spent years wondering how humanity would adjust to the anonymity of life in the city, the wrenching upheavals of mobile immigrant labor — a world of lonely people ripped from their social ties. We now have precisely the opposite problem. Indeed, our modern awareness tools reverse the original conceit of the Internet. When cyberspace came along in the early ’90s, it was celebrated as a place where you could reinvent your identity — become someone new.

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(Indeed, the question that floats eternally at the top of Twitter’s Web site — “What are you doing?” — can come to seem existentially freighted. What are you doing?) Having an audience can make the self-reflection even more acute, since, as my interviewees noted, they’re trying to describe their activities in a way that is not only accurate but also interesting to others: the status update as a literary form.