T he first time I joined Facebook, I had to quit again immediately. It was my first week of university. I was alone, along with thousands of other students, in a sea of club nights and quizzes and tedious conversations about other people's A-levels. This was back when the site was exclusively for students. I had been told, in no uncertain terms, that joining was mandatory. Failure to do so was a form of social suicide worse even than refusing to drink alcohol. I had no choice. I signed up.

Users of Facebook will know the site has one immutable feature. You don't have to post a profile picture, or share your likes and dislikes with the world, though both are encouraged. You can avoid the news feed, the apps, the tweet-like status updates. You don't even have to choose a favourite quote. The one thing you cannot get away from is your friend count. It is how Facebook keeps score.

Five years ago, on probably the loneliest week of my life, my newly created Facebook page looked me square in the eye and announced: "You have 0 friends." I closed the account.

Facebook is not a good place for a lonely person, and not just because of how precisely it quantifies your isolation. The news feed, the default point of entry to the site, is a constantly updated stream of your every friend's every activity, opinion and photograph. It is a Twitter feed in glorious technicolour, complete with pictures, polls and videos. It exists to make sure you know exactly how much more popular everyone else is, casually informing you that 14 of your friends were tagged in the album "Fun without Tom Meltzer". It can be, to say the least, disheartening. Without a real-world social network with which to interact, social networking sites act as proof of the old cliché: you're never so alone as when you're in a crowd.

The pressures put on teenagers by sites such as Facebook are well-known. Reports of cyber-bullying, happy-slapping, even self-harm and suicide attempts motivated by social networking sites have become increasingly common in the eight years since Friendster – and then MySpace, Bebo and Facebook – launched. But the subtler side-effects for a generation that has grown up with these sites are only now being felt. In March this year, the NSPCC published a detailed breakdown of calls made to ChildLine in the last five years. Though overall the number of calls from children and teenagers had risen by just 10%, calls about loneliness had nearly tripled, from 1,853 five years ago to 5,525 in 2009. Among boys, the number of calls about loneliness was more than five times higher than it had been in 2004.

This is not just a teenage problem. In May, the Mental Health Foundation released a report called The Lonely Society? Its survey found that 53% of 18-34-year-olds had felt depressed because of loneliness, compared with just 32% of people over 55. The question of why was, in part, answered by another of the report's findings: nearly a third of young people said they spent too much time communicating online and not enough in person.

In a YouGov poll published by Samaritans last December, 21% of young people aged 18-24 identified loneliness as one of their major concerns. Young people worried more than any other age group about feeling alone, being single, about the quality of their relationships with friends and family. Such figures have led newspapers to dub us the "Eleanor Rigby generation"; better connected than any in history, yet strangely alone.

Of course, twentysomethings are not the only ones using social networking sites but, unlike older users, we have developed and mediated many of our friendships through them. And the normality of digital communication has the odd effect of making us more distant from each other. "For older generations, who have come to it when their friendships are already well established, social networking just makes up for the fact that you can't be physically present all the time," says Mark Vernon, author of The Meaning Of Friendship. "You've spent plenty of time together in the past, and can understand the nuances of a short email or message. If you've mostly conducted your friendship online, you don't have that resource to draw on."

Not only is there a greater capacity for misunderstanding in online communication, we can come to view it as a substitute for genuine interaction. We become trapped in what Vernon calls the "tyranny of quantity" – exchanging dozens of short messages a day with a wide array of friends, rather than having one "quality" conversation. "Research done around BlackBerrys shows people use them in two ways: to keep close friends and family close – for example by sending a text saying, 'Just getting on the plane, love you' – and to keep a wider group of friends at a distance. If you send a quick message saying, 'Thanks for that, speak next week,' that's a distancing technique."

I suspect I'm not alone in having been through break-ups that took place entirely on Facebook. It's probably only a matter of time till I'm dumped via Twitter ("@tommeltzer It's over #itsnotyouitsme #itisyoureally"). The more removed the medium of communication, the easier it is for us to set aside our compassion. Reduce people to a single photo and a line of text, and suddenly they're far easier to ignore, or judge, or forget.

At the same time the reverse is also true. The distance between internet users allows us to stay intimately acquainted with the lives of people with whom we've long since ceased to communicate. Facebook stalking – poring over the messages and photographs of online friends – can make it hard for young people to move on as their social lives change. Several people I know make a daily pilgrimage to the profiles of their exes. In most cases this only serves to make them miserable.

The kind of voyeurism encouraged by sites such as Facebook can also make us feel inadequate. Will Ablett, a 23-year-old administrator from Portsmouth, quit the site last year. "I was unemployed for a while and it was my main form of communication with the outside world. I was sitting at home doing nothing and I felt frustrated seeing what everyone else was doing." He has now joined again, but uses the site very differently. "Being away from Facebook, I started texting and phoning, and meeting up with people. Now, with the people I am actually close to, all our contact is face-to-face."

This love/hate relationship with social networking is widespread. Ablett's experiences echo those of many others who find they simply cannot live without Facebook. To sign off from its pages is to excise yourself from social groups and invitations, to vanish from friends' lives. So we get stuck in a vicious circle, compulsively checking each other's status updates and feeling alienated as a result.

In fact, Vernon cites research carried out by David Holmes, a psychologist at Manchester Metropolitan university, who estimates that up to 40% of the information posted on social networking sites could be fabricated. This is partly, Vernon suggests, to protect privacy online, but there is also a desire to "present a side of ourselves rather than our whole selves". In this status-update culture, "we don't really live experiences, we live them to report them. We're editing ourselves rather than actually being ourselves." This alienates you not only from yourself but, ultimately, from those around you. "Rather than having a genuine encounter, your friends become your audience, and you are someone else's audience. The exchange is thwarted in both directions."

This breakdown in communication is bad for us mentally and physically. Studies show feeling alone makes it harder for us to regulate self-destructive habits, and weakens our cardiovascular and immune systems. A review last month of 148 studies on how relationships affect health found people had a 50% better survival rate if they were part of a wider group. Being lonely was as harmful as smoking 15 cigarettes a day or being an alcoholic.

Psychologist Dr Aric Sigman points out, "With loneliness comes a vast array of physiological risks and mental health problems." In an article for The Biologist, he described the growth of electronic media – computers, iPods, mobile phones, video games – as "the most significant contributing factor to society's growing physical estrangement".

But when the NSPCC asked counsellors for their own explanations for the rise in loneliness among young people, these ranged from lifestyle changes – families eating separately, longer working hours, the rise of divorce – to a positive cultural shift, with people becoming more open to talking about their feelings. Psychologist Kaveri Subrahmanyam, author of Adolescents On The Net: Internet Use And Well-being, says that while current research on social networking sites is sparse, it gives little cause for concern. "For the most part increased internet use appears to have given young people a means to interact with more people, and most of these people are from their offline lives. If you were already not lonely and had a good network of friends, your internet use would also lead to gains. I don't think online communities are replacing real-world social networks."

It is true that Facebook friends come mostly from real life. It would be wrong to say that in general they are our real-life friends. I have friends on Facebook I haven't seen since primary school. In several cases I have been added by people who, even as a nine-year-old, were at best an acquaintance, and in some cases an enemy. It's hard to see quite why I need to know that David, a man I remember primarily for beating me, aged seven, in a heated game of "Times Tables Shootdown", has just bought a new phone and is "luvin it". But know this I do. For every online friend that I regularly see or talk to, there are 25 more I will probably never speak to again. The friend count wins either way. Our digital communities come to resemble real-world neighbourhoods full of people who aren't even on speaking terms.

All of which is not to deny that these sites have their advantages. They are miraculously fast and efficient ways to share information. Thanks to Twitter I'd heard of Michael Jackson's death just minutes after it happened, giving me time to adjust my evening plans appropriately. Facebook has saved me from having to remember the dates of friends' birthdays, or even my own interests, and MySpace has made it easier than ever to pretend to like a friend's new band.

The online world may not be replacing our real-world social lives but it is not simply supplementing them, either. For boys in particular, online gaming masquerades as a way of socialising with people from the real world. I have several friends with whom I've communicated primarily over games of Modern Warfare. There are unlikely to be many social benefits to telling a friend, through an earpiece, to shoot down an attack helicopter. When I meet those people in the real world, we sometimes address each other, only half-ironically, by our Xbox Live usernames. Spend enough time watching a friend's avatar get its head shot off, or even just reading their tweets about "the MOST amazing sandwich", and your perception of them will change. Inevitably, our identities in digital space encroach on who we are in the real world.

Social networking has not, on its own, created a lonely generation. But nor has it done much to alleviate the problem. When The Lonely Society? report was published, Dr Mike Shooter of the Mental Health Foundation said, "There is no sadder sight than a young person left isolated in a bedsit, feeling miserable and picked on by those who may have similar self-doubts but who cover them up by shunning those who seem weaker than themselves." Perhaps there is, though: that same young person, sitting at a computer and writing a message about a sandwich, to no one in particular.