There are more than 300 million of us in the United States, and sometimes it seems like we’re all friends on Facebook. But the sad truth is that Americans are lonelier than ever. Between 1985 and 2004, the number of people who said there was no one with whelm they discussed important matters tripled, to 25 percent, according to Duke University researchers. Unfortunately, as a new study linking women to increased risk of heart disease shows, all this loneliness can be detrimental to our health. The bad news doesn’t just affect women. Social isolation in all adults has been linked to a raft of physical and mental ailments, including sleep disorders, high blood pressure, and an increased risk of depression and suicide. How lonely you feel today actually predicts how well you’ll sleep tonight and how depressed you’ll feel a year from now, says John T. Cacioppo, a neuroscientist at the University of Chicago and coauthor of Loneliness: Human Nature and the Need for Social Connection . Studies have shown that loneliness can cause stress levels to rise and can weaken the immune system. Lonely people also tend to have less healthy lifestyles, drinking more alcohol, eating more fattening food, and exercising less than those who are not lonely. Though more Americans than ever are living alone (25 percent of U.S. households, up from 7 percent in 1940), the connection between single-living and loneliness is in fact quite weak. "Some of the most profound loneliness can happen when other people are present," says Harry Reis, professor of psychology at the University of Rochester. Take college freshmen: even though they’re surrounded by people almost all the time, many feel incredibly isolated during the first quarter of the school year with their friends and family members far away, Cacioppo says. Studies have shown that how lonely freshmen will feel can be predicted by how many miles they are from home. By the second quarter, however, most freshmen have found social replacements for their high-school friends. Unfortunately, as we age, it becomes more difficult to recreate those social relationships. And that can be a big problem as America becomes a more transient society, with an increasing number of Americans who say that they’re willing to move away from home for a job. Loneliness can be relative: it has been defined as an aversive emotional response to a perceived discrepancy between a person’s desired levels of social interaction and the contact they’re actually receiving. People tend to measure themselves against others, feeling particularly alone in communities where social connection is the norm. That’s why collectivist cultures, like those in Southern Europe, have higher levels of loneliness than individualist cultures, Cacioppo says. For the same reason, isolated individuals feel most acutely alone on holidays like Christmas Eve or Thanksgiving, when most people are surrounded by family and friends. Still, loneliness is a natural biological signal that we all have. Indeed, loneliness serves an adaptive purpose, making us protect and care for one another. Loneliness essentially puts the brain on high alert, encouraging us not to eat leftovers from the refrigerator but to call a friend and eat out. Certain situational factors can trigger loneliness, but long-term feelings of emptiness and isolation are partly genetic, Cacioppo says. What’s inherited is not loneliness itself, but rather sensitivity to disconnection. Social-networking sites like Facebook and MySpace may provide people with a false sense of connection that ultimately increases loneliness in people who feel alone. These sites should serve as a supplement, but not replacement for, face-to-face interaction, Cacioppo says. He compares connecting on a Web site to eating celery: "It feels good immediately, but it doesn’t give you the same sustenance," he says. For people who feel satisfied and loved in their day-to-day life, social media can be a reassuring extension. For those who are already lonely, Facebook status updates are just a reminder of how much better everyone else is at making friends and having fun. So how many friends do you need to avoid loneliness There’s no magic number, according to Cacioppo. An introvert might need one confidante not to feel lonely, whereas an extrovert might require two, three, or four bosom buddies. Experts say it’s not the quantity of social relationships but the quality that really matters. "The most popular kid in school may still feel lonely," Cacioppo says. "There axe a lot of stars who have been idols and lived lonely lives." The author’s main purpose of mentioning Facebook is toA. compare Facebook with face-to-face communication. B. emphasize the supplementary function of Facebook to daily communication. C. illustrate the importance of Facebook in helping people to relieve loneliness. D. introduce the ways to make friends on Facebook.