Sending holiday greeting cards began in the 1850s in England and spread quickly as a way to stay in touch with far-flung friends and relatives. The cards, whether religious or not in theme, went to people you rarely wrote to and even more rarely spoke to, but for whom you still had a measure of affection — or curiosity. You wanted to know what was going on in their lives, and one exchange a year did the trick.

The cards kept the people in your social network at a distance, while maintaining ties to them. I recall my parents sending and receiving Christmas cards. I did it for a year after I married, but I stopped because it was just too much work.

Facebook, which tries to replicate our real-world relationships online, now helps me maintain those connections. But it does cards one better. It preserves the weak ties in my social network without creating obligations.

A problem I have with holiday cards is that receiving one creates an obligation to send one back. If you don’t, you can expect to be dropped from the friend’s mailing list. You have to send a card next year to re-establish the ties, fully expecting not to get a card in return because of the earlier faux pas. (I do still get four cards each December from non-obligation-seeking friends — and enjoy each one.)

There is no exact way to measure the decline of holiday cards. But the overall drop in the amount of December mail might serve as an approximate indicator. The United States Postal Service reports that the December volume of first-class mail — no catalogs, packages or junk mail — fell 18 percent from a peak in 2002.

Facebook, meanwhile, has grown to 500 million users worldwide and continues to find new users. And Facebook says it finds it hard to think that anyone would want to use their service to keep people away. And yet, last week, it introduced a new service that allows people to divvy up their friends into cliques, a tacit acknowledgment that we hold most of our Facebook friends at a distance.

But it is the lack of obligations that makes Facebook better than other forms of communication. It’s socially efficient and even appears to make staying in touch a whole lot less work. You can read about friends taking their children to soccer games, but your lack of response doesn’t mean that you will stop hearing about their birthday parties, swim lessons, prom nights and graduations.

You can even hide a friend’s messages and they are none the wiser. Facebook also lets people create hierarchies of friends so that information can be shared selectively. No one but you sees the cliques you assign people to. It shouldn’t be too surprising that Facebook has this effect. After all, technology has a grand tradition of distancing people from one another for the sake of efficiency.

The automated gas pump ensures that you no longer interact with a station attendant unless you have a hankering for a slushie and a Slim Jim. The bank A.T.M. has stopped us from exchanging pleasantries with the teller as we collect our $20 bills. And supermarkets are constantly experimenting with self-checkout technology that will remove the line-slowing interaction with a human clerk from the process.

Talking on the phone replaced face-to-face meetings. Now texting is taking the place of many phone conversations. But those calls may not be missed any more than we now miss talking to bank tellers, as they often require small talk and can be awkward to break off. For the recipients, they are an interruption.
(Of course, Apple is advertising face-to-face video phone calls with the same kind of emotion that AT&T used in its old “reach out and touch someone” campaigns. But I suspect that calls are a lost cause.)
Despite its ability to conveniently hold friends at a distance, Facebook can also bestow benefits to those users who make a little effort to keep track of what their friends are saying there. Cameron Marlow, a research scientist who has been described as Facebook’s in-house sociologist, says it relates to social capital: Your friends on Facebook have resources — a guest room or a job, for instance. You may want to know that information, but finding it depends in no small amount on serendipity and your ability to pay attention to their posts. But you also have to be willing to ask for those resources.

Mr. Marlow says that more in-bound communication increases one’s social capital. “The more they have communication with me, the lower the cost of me reaching out to them,” he said.

And a little reciprocity increases the amount of information we receive. The new feature, Facebook groups, allows like-minded friends to more easily share information.

EVEN people who aren’t very sociable on Facebook can benefit from the knowledge they gain about their friends. “You look them up; you see what they do,” Mr. Marlow says. “Six months later you could reach out. There is no social norm against reaching out.”

There was a day, he said, when “birthday or Christmas cards were the only way to reach out and say I care about you,” said Mr. Marlow. But now, whether you hold friends close or not, you gain timely and useful information as it trickles out, and not in a once-a-year data dump.