We all need friends. We always have. Our ancestors found obtaining the basic necessities of life was easier in a group. But can you can have too much of a good thing? It’s estimated that the maximum number of people who lived in early communities of hunter-gatherers was 150.

According to evolutionary anthropologist Robin Dunbar, 150 is the largest number of people you can share trust and obligations with. This value is known as ‘Dunbar’s number’. It is thought to be a cognitive limit to the number of friends we can maintain, not the number of people we know. People can boast thousands of ‘friends’ on Facebook, but Dunbar would say that it is impossible to feed and nourish all these relationships. ‘I have 293 friends on Facebook,’ says John, 36. ‘I count 280 as close – hard to believe, actually.’

Social networking sites such as Facebook have changed the landscape of friendship. It may be the case that the ease with which we can now remain in touch makes Dunbar’s number less relevant. There may well be limits to the number of people we can keep a quality friendship with, the type of friend we phone for advice and help. But it may be that we are able to maintain, at a lower level, additional friendships of a different quality far beyond this number. Yet how many of this number could you count on in your hour of need? On Christmas Day, Simone Back, 42, from Brighton, announced on Facebook to her 1,082 friends: ‘Took all my pills be dead soon so bye bye everyone.’ There were 150 online responses, including taunts and bickering. No one who lived nearby tried to help her or called 999. Her body was found the next day.

Part of the problem is that the meaning of the word ‘friend’ remains somewhat unclear. While Juliette, 43, regards a friend as someone who provides ‘emotional support both ways’, Mark, 40, considers a friend ‘someone whose life has crossed mine’. Dunbar’s number might broadly describe the number of people with whom we share trusts and obligations, but it may underestimate the number of people with whom we might associate – whether sharing a drink with someone at the water cooler, or a glass of wine at your sister’s wedding. Our circle of associates includes work colleagues, people we meet at parties, in the pub, on the train to work.

Some associates are much more important than others – our family, our closest confidants and our children. Some are less important – the girls from our old netball team, the people in the office downstairs and Michael from marketing, with whom we went to the cinema once last year. We are capable of exchanging an email with all these people, and may have mutual acquaintances with a good number of them, providing common ground. When we think about it for a moment, the single label of ‘friend’ is not terribly useful. The quality and depth of our friendships is more important.

The difference in definitions of friendship often boils down to a question of needs. For some, friendship is all about affiliation – the need to have a large number of positive connections. For others, friendship is more about intimacy – the need to have a few warm, close relationships. Having lots of affiliates does have its advantages. ‘A week before travelling, I posted on Facebook: “I’m in New York next week, anyone around?”’ says James, 23. ‘I met up with two old school friends who’d moved there 18 months earlier. It makes travel more random, more interesting.’

While both affiliation and intimacy are rewarding, in the long run, quality makes a bigger difference than quantity. Research suggests that people who choose to stick with a few close confidants in youth are better adjusted by middle age than those who need to chum up with many. Social networking is a minefield for those with a strong need to affiliate. While it opens the opportunity to connect with the multitude, it also invites the possibility of rejection, where people can be ‘defriended’ by existing connections, or friendship requests can be turned down by prospective ones. People who need to affiliate are more likely to find these rejections particularly crushing. Some people find that social networking contributes to a sense of loneliness and inadequacy. An anonymous blogger wrote: ‘If I post something and no one responds, I feel sad and lonely. If I go on a friend’s page and see that they gave a rose or some other gift to some mutual friends, I wonder why I didn’t get anything. I feel like reaching out, but fear I will come across as needy or emotionally bereft.’

Degrees of comfort with using Facebook as a forum for friendship depend on your generation. In one camp are ‘digital natives’ – those born during or after the introduction of digital technology. They grew up with mobile phones, emails, blogs. When I ask digital native Ashley, 22, to describe someone who doesn’t use Facebook, she looks utterly surprised. ‘Everyone is on Facebook,’ she says, flatly. Tell a digital native you are not on Facebook and you can expect the response, ‘How do you live without Facebook?’

In the other camp are digital immigrants. They were born before the advent of digital technology, and are likely to have adopted it – to some extent, by necessity – later in life. They include the ‘avoiders’, who are suspicious of most social networking sites. They see the phenomenon as exhibitionist, voyeuristic, a threat to personal privacy. ‘It is pointless to collect friends you have lost touch with from your past,’ says Adam, 41. ‘If you aren’t good friends, it is probably for a good reason.’

Digital immigrants also count among their number the reluctant adopters who are likely to go on Facebook because they feel they have to, but try their best to avoid it. Friends aren’t bound by blood or family bonds, employment contracts or legal obligations. We do things for one another because we want to. Whether writing a response to a photo tagged on Facebook, sending a birthday card by post, or making a hospital visit, the efforts we make for our friends are intentional acts of kindness. It is this voluntary nature of friendship that makes it rewarding – and precious.

In short-term or less intimate relationships, we are more aware of the importance of repaying favours quickly. For our closer friendships it is less tit for tat, quid pro quo – we come to trust that things will balance out in the end and that you will be there for one another when it matters most. We feel good about helping our friend out because, well, we are friends. Friendships take work, they need to be nurtured, nourished and maintained. We often let our friendships slide because of a lack of time and energy. When carrying a heavy load at work, involved in a serious relationship or married with children, it’s hard to find time for friends. In these situations we need to remind ourselves of what friends are for and why they can be and very often are vital in our lives. They save us from being overly dependent on one relationship for our identity. Even more than a buffer against what life can throw at us, friends are a source of personal enrichment and growth. In a survey of more than 2,000 married couples aged 55 or older, for both men and women, having friends was the best predictor of being satisfied in marriage – probably because they were more satisfied with their lives as a result of having friends.

Researchers at UCLA suggest social relationships are the reason women live longer than men. Social bonds reduce the risk of disease by decreasing blood pressure, cholesterol and heart rate. A lack of close confidants is as harmful to your health as smoking or being overweight. Indeed, people without friends are more likely to die younger. Not only do friendships prolong our lives, they make us happier, too. People who are socially active are less stressed and depressed – they also tend to feel better about their lives and who they are. It is estimated that by 2020, one billion people will be on Facebook. Whether you use it or not, it’s worth remembering, friendship is about seeing eye to eye, not counting heads.

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Read Sex, lies and social media by David Head on LifeLabs