The personal quandaries thrown up by social-networking sites seem to be escalating by the day. What do you do when, say, a work colleague - whom you see across the office but with whom you never exchange more than courteous pleasantries - asks you to become a friend on Facebook? Your policy to date has been that your profile is strictly for real friends only. But can you risk the icy stares should you refuse him and click "ignore"?

Or there's the question of what you should upload onto such sites. Simon Davies, the director of Privacy International, plays a neat trick to make a point. There are some who argue that if you have nothing to hide, you have nothing to fear about whatever gets into the ever-expanding online data- warehouses of MySpace and other social networks. So before engaging them in debate, Davies finds something "private" about them - for example, the email of a daughter. With Google, it is amazingly easy to do. It is also amazing how quickly people change their mind.

What's mine is the world's

There are two reasons to think carefully about this near-limitless ease of access to personal information. First, research shows that social-networking sites are a serious risk when accessed at work. "Companies are split on the question of Facebook", says Graham Cluley, senior technology consultant at Sophos. "Some believe it to be a procrastinator's paradise which can lead to identity theft if users are careless. Others either view it as a valuable networking tool for workers or are too nervous of an employee backlash if the site is suddenly blocked." He advocates the teaching of best practices - "how to use

Facebook at work". It may sound ridiculous, but the purpose is to ensure that employees are not putting their personal and corporate data out to tender. "Five minutes spent learning the ins-and-outs of Facebook's privacy settings, for instance, could save a lot of heartache later", Cluley adds.

Mark Vernon is the author of The Philosophy of Friendship (Palgrave Macmillan, 2005) and Science, Religion and the Meaning of Life (Palgrave Macmillan, 2007). His new book explores online friendship among other topics: What Not To Say: Finding the Right Words at Difficult Moments (Orion, November 2007). His website is here

Also by Mark Vernon in openDemocracy:

"The politics of friendship" (29 December 2006)

"The life of the child: being friends, being good" (8 March 2007)

The second reason is that once uploaded, personal details can become public possession - and not just for now but, effectively, forever. News Corp bought MySpace to exploit what previously had been unthinkable to advertisers: customers telling you what they want without you even asking.

The latest twist in the story of citizens "handing over" the details of their lives relates to the burgeoning "family history" industry. The website ancestry.com is offering a new service: testing your DNA online. Ostensibly it will put you in touch with other family members. But Simon Davies has made a complaint to the office of the British government's information commissioner on privacy grounds. He points out, for example, that ancestry.com's privacy policy describes genetic and genealogical data held by it as a "transferable asset"; "Does this mean that ancestry.com regards itself as the ‘owner' of a person's DNA?", Davies asks.

This is new terrain. In the pre-internet era, a chat with friends remained a private matter. On the internet, there are few safeguards against what will enter the public domain.

I've got a friend

This raises a further, equally "intimate" question of another kind: what is this doing to friendship itself? Will Reader of Sheffield Hallam University told the British Association Festival of Science in September 2007 that the number of online casual relationships on "virtual nodding terms" is rocketing. "To have in excess of 1,000 friends is not uncommon", Reader explained. "It can be a bit like trainspotting. They just want to get as many people onto their list as possible. It does upset some people. They start by feeling good that they appear to have made a new friend only to find out that they are simply being added to a list. They're not wanted for themselves; they're wanted to extend a list."

An evocative expression I heard the other day seems about right as a description of life online: "pacing the border between solipsism and communication". The issue is quantity. With the internet, the computer has become a social hub that knows no geographical limit. "Hooray!", shout the cheerleaders. "When it comes to friendship more is always more." That might have been the case when you were meeting new folk at the parents' evening or in the local church. But when you are talking acquaintances in their hundreds, let alone thousands, it rapidly becomes unmanageable - amity-wise at least. A sharper way of putting it is that this magnificent communications medium struggles most precisely at the moment when people really try to communicate!

It is little wonder then that with remorseless logic, the communicative non-communication of social-networking sites includes the new phenomenon of "defriending". This is when those virtual "nods" that were once reciprocated are subsequently ignored. "Normally a friendship will fade out", Will Reader's contribution at the science festival continued. "You gradually lose contact. On these sites you remove them. It's a type of spring-clean and the other person knows they've been removed."

The irony in all this is that Facebook - which in September 2007 overtook MySpace in Britain as the preferred site for individual users - was originally set up to mirror rather than overturn the "intimacy" and exclusiveness of real-world, face-to-face networks. Andrew McCollum, one of the founders of Facebook, explained to me that they based the project on a pretty closed community, namely university colleges. "They are second only to prisons in terms of being large groups of people in which everyone sees each other day in, day out",' he said. The significance of this is that the connections between people represented online started in real life and then went online - not the other way. "It is based upon providing nothing more than a utility, like a very big directory or self-sustaining, multi-dimensional telephone book", McCollum continued.

This perhaps explains why it turns out that the number of close friends people say they have has remained roughly the same as it ever was: somewhere between six and a dozen. Ray Pahl, co-author of Rethinking Friendship: Hidden Solidarities Today, says: "Technology makes it easier to make friends and contact people than ever before, but this brings a new set of problems. Our research reveals that even the most well-connected can feel overwhelmed and anxious as they struggle to ‘service' their friends."

Pahl has recommended a "friendship health check" to help you get on top of your relationships. I myself am experimenting with a similar possibility, having set up a "friendship intelligence test" online. There seems to me to be little point in being prescriptive about it. But a little thought about what you seek in friendship, and whether the internet is the place to go for it, might go a long way - and save some of that heartache.

As Aristotle, the great philosopher of friendship, might have said: "It is not that the internet is inherently bad for friendship. It is just not a very skilful way to do it."