“It’s like I can distantly read everyone’s mind,” Haley went on to say. “I love that. I feel like I’m getting to something raw about my friends. It’s like I’ve got this heads-up display for them.” It can also lead to more real-life contact, because when one member of Haley’s group decides to go out to a bar or see a band and Twitters about his plans, the others see it, and some decide to drop by  ad hoc, self-organizing socializing. And when they do socialize face to face, it feels oddly as if they’ve never actually been apart. They don’t need to ask, “So, what have you been up to?” because they already know. Instead, they’ll begin discussing something that one of the friends Twittered that afternoon, as if picking up a conversation in the middle.

Facebook and Twitter may have pushed things into overdrive, but the idea of using communication tools as a form of “co-presence” has been around for a while. The Japanese sociologist Mizuko Ito first noticed it with mobile phones: lovers who were working in different cities would send text messages back and forth all night  tiny updates like “enjoying a glass of wine now” or “watching TV while lying on the couch.” They were doing it partly because talking for hours on mobile phones isn’t very comfortable (or affordable). But they also discovered that the little Ping-Ponging messages felt even more intimate than a phone call.

“It’s an aggregate phenomenon,” Marc Davis, a chief scientist at Yahoo and former professor of information science at the University of California at Berkeley, told me. “No message is the single-most-important message. It’s sort of like when you’re sitting with someone and you look over and they smile at you. You’re sitting here reading the paper, and you’re doing your side-by-side thing, and you just sort of let people know you’re aware of them.” Yet it is also why it can be extremely hard to understand the phenomenon until you’ve experienced it. Merely looking at a stranger’s Twitter or Facebook feed isn’t interesting, because it seems like blather. Follow it for a day, though, and it begins to feel like a short story; follow it for a month, and it’s a novel.

You could also regard the growing popularity of online awareness as a reaction to social isolation, the modern American disconnectedness that Robert Putnam explored in his book “Bowling Alone.” The mobile workforce requires people to travel more frequently for work, leaving friends and family behind, and members of the growing army of the self-employed often spend their days in solitude. Ambient intimacy becomes a way to “feel less alone,” as more than one Facebook and Twitter user told me.

When I decided to try out Twitter last year, at first I didn’t have anyone to follow. None of my friends were yet using the service. But while doing some Googling one day I stumbled upon the blog of Shannon Seery, a 32-year-old recruiting consultant in Florida, and I noticed that she Twittered. Her Twitter updates were pretty charming  she would often post links to camera-phone pictures of her two children or videos of herself cooking Mexican food, or broadcast her agonized cries when a flight was delayed on a business trip. So on a whim I started “following” her  as easy on Twitter as a click of the mouse  and never took her off my account. (A Twitter account can be “private,” so that only invited friends can read one’s tweets, or it can be public, so anyone can; Seery’s was public.) When I checked in last month, I noticed that she had built up a huge number of online connections: She was now following 677 people on Twitter and another 442 on Facebook. How in God’s name, I wondered, could she follow so many people? Who precisely are they? I called Seery to find out.

“I have a rule,” she told me. “I either have to know who you are, or I have to know of you.” That means she monitors the lives of friends, family, anyone she works with, and she’ll also follow interesting people she discovers via her friends’ online lives. Like many people who live online, she has wound up following a few strangers  though after a few months they no longer feel like strangers, despite the fact that she has never physically met them.

Image Credit... Peter Cho

I asked Seery how she finds the time to follow so many people online. The math seemed daunting. After all, if her 1,000 online contacts each post just a couple of notes each a day, that’s several thousand little social pings to sift through daily. What would it be like to get thousands of e-mail messages a day? But Seery made a point I heard from many others: awareness tools aren’t as cognitively demanding as an e-mail message. E-mail is something you have to stop to open and assess. It’s personal; someone is asking for 100 percent of your attention. In contrast, ambient updates are all visible on one single page in a big row, and they’re not really directed at you. This makes them skimmable, like newspaper headlines; maybe you’ll read them all, maybe you’ll skip some. Seery estimated that she needs to spend only a small part of each hour actively reading her Twitter stream.