The much-hyped arrival of the Google+ social networking site throws open the debate on the difference between friends and acquaintances. So where is the line?

"If a man does not make new acquaintance as he advances through life, he will soon find himself left alone," said Samuel Johnson. "A man should keep his friendships in constant repair."

The comment takes on new meaning in the Facebook era when it's common for people to have hundreds of "friends". But they're not friends in the conventional sense.

The main novelty of Google+ is that it requires users to arrange their contacts in different categories or "circles". A new member is given the default settings of Friends, Family or Acquaintance and encouraged to create new circles under headings of their own choosing.

Facebook aficionados would point out that it has always been possible to create groups on Facebook, but some can find it slightly fiddly. Google+ seems to push the user down this route so that they target messages at particular contacts.

"I can see the logic of Google's split. Friends and family are quite insulated groups from each other, who you relate to in different ways," says Prof Robin Dunbar, author of How Many Friends Does One Person Need?

He argues that beyond a core group of 150 people - made up of "intimates", family members, and friends - what one is really talking about are "nodding acquaintances".

One list is for people I can tolerate talking to. The other is for people who can fall down a well. Caitlin Moran,, Times columnist

The phone-hacking scandal, with its nexus of journalists, media owners, politicians and police officers, has thrown a spotlight on the dividing line.

John Yates, who this week resigned as assistant commissioner of the Metropolitan Police, referred to former News of the World executive Neil Wallis as a "friend" but not a "bosom buddy". David Cameron has been happy to call his former media adviser Andy Coulson a "friend". But some feel the need to offer a qualification.

Jeff Jarvis, who writes the BuzzMachine blog, says the distinction between an acquaintance or friend doesn't matter online. "Acquaintance is just a label. I don't use it."

The circles on Google+ are just modern day mailing lists, he says. He organises his Google+ contacts into circles labelled things like "Germany", "World" or "Celebrity".

He argues it's about relevance rather than privacy or intimacy. "The idea you can use these sites as a vault for your private thoughts is absurd. If you have a secret, leave it in your head."

The different social networks overlap but prioritise different things. Indeed Facebook already allows users to put people into groups, it's just a "pain" doing so, says Jarvis. Facebook is for maintaining relationships, Twitter for broadcasting, Tumblr for quotations and Google+ for sharing, he believes.

Image caption Facebook allows people to have thousands of friends

Times columnist Caitlin Moran is an early adopter of Google+. She has never been a fan of Facebook - "too slow" - and although previously enthusiastic about Twitter says it has become too nasty. "Twitter is like a pub full of people shouting. Google+ is more like withdrawing to the smoking room with friends."

Moran has eschewed Google+'s ability to have numerous different circles in favour of a clear distinction. After importing contacts from her existing social networks she set up two circles. "One is for people I can tolerate talking to. The other is for everyone else who can fall down a well."

But in the Facebook age, it's easy to get confused about friendship, says Gladeana McMahon, the former GMTV life coach. "A friend is someone you've got a regular relationship with now or in the past. You keep in contact and are involved in their life."

On the other hand, an acquaintance is someone you've met but not had the chance to develop a friendship with. It may turn into friendship. But often it will never go further than a brief hello.

You can create worthwhile relationships online, she says. If they're meaningful they will probably move into the "real" world but it's also possible to have a modern version of the "pen pal" arrangement using Facebook or Skype to talk to someone you'll never meet in the flesh.

But social networks will never be able to understand the multiplicity of human relationships, she warns. "One thing that will never fit into a box is a human being."

Jarvis says that Mark Zuckerberg grasped this point. The Facebook creator believes that people each have one authentic identity rather than different selves. In interviews for Jarvis's forthcoming book Public Parts, Zuckerberg told him that it was becoming impossible for people to maintain separate identities.

A classical view Cicero's On Friendship (44 BC) • "A friend is, as it were, a second self" • "The shifts of Fortune test the reliability of friends" • "Friendship makes prosperity more shining and lessens adversity by dividing and sharing it" • "Many pecks of salt must be eaten before the duties of friendship can be discharged"

Essentially our work, home, recreational and family selves are one and the same. If you bump into your lawyer at the supermarket at the weekend, he may be wearing shorts and be with his child. He is a complete person not just someone you know from work, Jarvis argues.

Dunbar, professor of evolutionary anthropology at Oxford University, says that may work while you're at university. But by the end of one's twenties it's no longer tenable. Not only do we see people in different contexts, like work or leisure, but we also have friends scattered around the world who don't necessarily mix or interact.

And then there's how we communicate online. Status updates on Facebook are often banal, he says. "You write 'I had Weetabix for breakfast', which your best friend may care about but not many others will."

This is because people don't change the way they converse online. Research has shown that they behave as if they are addressing a maximum of three other people, even though a post may go out to hundreds.

And social networking friendships demand a huge amount of work if they are to be maintained. Otherwise they will wither in a matter of months once emotional closeness is lost, Dunbar says. When it comes to using technology with our closest friends, it is texts not websites that count, he argues.

Research carried out by one of his postdoctoral students involving 30 Sheffield sixth formers showed that 85% of the texts they sent were going to one of two people - usually a boyfriend or girlfriend and a best mate. It illustrates that there are things we still want to share with only our nearest and dearest, he says.

But as Oscar Wilde reminds us, friendship mixed with human nature can be a prickly business. "Anybody can sympathise with the sufferings of a friend, but it requires a very fine nature to sympathise with a friend's success." Or, for that matter, their choice of breakfast.