Online dating is now one of the most common ways to start a relationship. But is it fulfilling our dreams – or shattering our cherished ideal of romance?

'I'm telling you, this is Love Year Zero, the Year of True Love, the Real Thing." So writes cinderella69 (AKA Jennifer – she was born in 1969) in her blog about her online dating experiences. "You couldn't do this until now. You went on waiting and waiting for your Prince, and you still had a long wait ahead of you, because he didn't know you were waiting, poor thing. Now you're on the net, and everyone knows it. It can't fail to work. All you have to do is look."

She's right. Last millennium 72% of us met our partners at school or university, at work or in networks of family or friends. The other 28%, presumably, met the loves of their lives by tripping over them as they lay in their own filth outside a Black Country pub. Or such were mating rites in my day. The internet is revolutionary because it renders it easy for us to make contact with people we don't know and, better yet, those who don't necessarily live within the Dudley travel-to-work area/look like trolls/cite assembling Airfix models as their favourite hobby even though they're 43, etc.

According to a new survey by psychologists at the University of Rochester in the US, online dating is the second most common way of starting a relationship – after meeting through friends. It has become popular in part, says one of the report's authors, Professor Harry Reis, because other methods are widely thought of as grossly inefficient. "The internet holds great promise for helping adults form healthy and supportive romantic partnerships, and those relationships are one of the best predictors of emotional and physical health," he says.

The Guardian, for example, has had its own and very successful online dating site, Soulmates, since 2004 – more than 650,000 have registered. It can put you in touch with Guardian readers – true, that may be some people's worst nightmare, but it does mean you won't get propositioned online by someone whose leisure activities are attending English Defence League demos and you won't have to explain on a date that Marcel Proust wasn't an F1 racing driver.

Online dating offers the dream of removing the historic obstacles to true love (time, space, your dad sitting on the porch with a shotgun across his lap and an expression that says no boy is good enough for my girl). And online dating sites, which in the US are growing at 70% a year, surely make it easier than ever to meet the Prince Charming or the Fairytale Princess of your, frankly, infantile dreams. At least that's what cinderella69 believes.

But she's also wrong: it often fails to work – not least because elsewhere in cyberspace there are people like Nick, who aren't looking for love from online dating sites, but for sexual encounters as perishable and substitutable as yoghurt. In his sex blog, Nick works out that he got 77.7% of the women he has met through online dating sites into bed on the first night, and that 55% of his dates were "one-offs", three were "frigid", two were "not too great", eight "hot" and two "atomic". I know, I know: who'd have thought atomic sex was desirable rather than a trip to A&E waiting to happen? Thanks to the internet, such spreadsheets of love have replaced notches on the bedpost and can be displayed hubristically online.

But there's another problem for the lie-dream of online romantic fulfilment: in the hypermarket of desire, as in a large Tesco's breakfast cereal aisle, it's almost impossible to choose.

"When you look at their profiles, they're all the same," wails channelchris in her blog. "Charming, sporty, generous, funny, 'no mind games', good-looking, sensual ... They practically guarantee you'll be on cloud nine."

When everyone is presenting themselves as practically perfect in every way, then you're bound to worry you've signed up for a libido-frustrating yawnathon.

The foregoing sex bloggers are quoted by Sorbonne sociologist Jean-Claude Kaufmann in his new book Love Online, in which he reflects on what has happened to romantic relationships since the millennium. The landscape of dating has changed completely, he argues. We used to have yentas or parents to help us get married; now we have to fend for ourselves. We have more freedom and autonomy in our romantic lives than ever and some of us have used that liberty to change the goals: monogamy and marriage are no longer the aims for many of us; sex, reconfigured as a harmless leisure activity involving the maximising of pleasure and the minimising of the hassle of commitment, often is. Online dating sites have accelerated these changes, heightening the hopes for and deepening the pitfalls of sex and love.

"I've been researching love and coupledom for 30 years and now the internet has brusquely changed the rules of dating," Kaufmann tells me. "Love isn't an eternal given – it evolves with societies. And people want to know how it functions now. It's urgent to analyse it."

Kaufmann isn't the only intellectual analysing the new landscape of love. Behavioural economist Dan Ariely is researching online dating because it affects to offer a solution for a market that wasn't working very well. Oxford evolutionary anthropologist Robin Dunbar will soon publish a book called The Science of Love and Betrayal, in which he wonders whether science can helps us with our romantic relationships. And one of France's greatest living philosophers, Alain Badiou, is poised to publish In Praise of Love, in which he argues that online dating sites destroy our most cherished romantic ideal, namely love.

Ariely started thinking about online dating because one of his colleagues down the corridor, a lonely assistant professor in a new town with no friends who worked long hours, failed miserably at online dating. Ariely wondered what had gone wrong. Surely, he thought, online dating sites had global reach, economies of scale and algorithms ensuring utility maximisation (this way of talking about dating, incidentally, explains why so many behavioural economists spend Saturday nights getting intimate with single-portion lasagnes).

Online dating is, Ariely argues, unremittingly miserable. The main problem, he suggests, is that online dating sites assume that if you've seen a photo, got a guy's inside-leg measurement and star sign, BMI index and electoral preferences, you're all set to get it on à la Marvin Gaye, right? Wrong. "They think that we're like digital cameras, that you can describe somebody by their height and weight and political affiliation and so on. But it turns out people are much more like wine. When you taste the wine, you could describe it, but it's not a very useful description. But you know if you like it or don't. And it's the complexity and the completeness of the experience that tells you if you like a person or not. And this breaking into attributes turns out not to be very informative."

So he decided to set up a website that could better deliver what people want to know about each other before they become attracted. His model was real dates. "Dates are not about sitting in the room and interviewing each other about questions; they're often about experiencing something together in the real world. If you and I went out, and we went somewhere, I would look at how you react to the outside world. What music you like, what you don't like, what kind of pictures you like, how do you react to other people, what do you do in the restaurant. And through all these kind of non-explicit aspects, I will learn something about you."

His online system gave visitors an avatar with which to explore a virtual space. "There were pictures and images and there were words and movies and bands, all kinds of stuff, and when you came to another little avatar, you could start chatting. It wasn't about where you went to school and what's your religion; it was about something else, and it turns out it gave people much more information about each other, and they were much more likely to want to meet each other for a first date and for a second date."

Badiou found the opposite problem with online sites: not that they are disappointing, but they make the wild promise that love online can be hermetically sealed from disappointment. The septuagenarian Hegelian philosopher writes in his book of being in the world capital of romance (Paris) and everywhere coming across posters for Meetic, which styles itself as Europe's leading online dating agency. Their slogans read: "Have love without risk", "One can be in love without falling in love" and "You can be perfectly in love without having to suffer".

Badiou worried that the site was offering the equivalent of car insurance: a fully comp policy that eliminated any risk of you being out of pocket or suffering any personal upset. But love isn't like that, he complains. Love is, for him, about adventure and risk, not security and comfort. But, as he recognises, in modern liberal society this is an unwelcome thought: for us, love is a useless risk. "I really think that love, in our world such as it is, is encircled, threatened. And I think it's a philosophical task, among others, to defend it."

Across Paris, Kaufmann is of a similar mind. He believes that in the new millennium a new leisure activity emerged. It was called sex and we'd never had it so good. He writes: "As the second millennium got underway the combination of two very different phenomena (the rise of the internet and women's assertion of their right to have a good time), suddenly accelerated this trend ... Basically, sex had become a very ordinary activity that had nothing to do with the terrible fears and thrilling transgressions of the past." Best of all, perhaps, it had nothing to do with marriage, monogamy or motherhood but was devoted to enjoyment, to that scarcely translatable (but fun-sounding) French word jouissance.

Thanks to online dating sites, Kaufmann suggests, "there was now a vast hypermarket for love and/or sex, in which everyone was both a buyer and seller who openly stated what they wanted and tried to satisfy their needs as efficiently as possible. All they needed to do was sign up, pay a modest fee (getting a date costs less than going to see a film), write a blog or use a social networking site. Nothing could be easier."

In a sense, though, sex and love are opposites. One is something that could (but perhaps shouldn't) be exchanged for money or non-financial favours; the other is that which resists being reduced to economic parameters. The problem is that we want both, often at the same time, without realising that they are not at all the same thing. And online dating intensifies that confusion.

Take sex first. Kaufmann argues that in the new world of speed dating, online dating and social networking, the overwhelming idea is to have short, sharp engagements that involve minimal commitment and maximal pleasure. In this, he follows the Leeds-based sociologist Zygmunt Bauman, who proposed the metaphor of "liquid love" to characterise how we form connections in the digital age. It's easier to break with a Facebook friend than a real friend; the work of a split second to delete a mobile-phone contact.

In his 2003 book Liquid Love, Bauman wrote that we "liquid moderns" cannot commit to relationships and have few kinship ties. We incessantly have to use our skills, wits and dedication to create provisional bonds that are loose enough to stop suffocation, but tight enough to give a needed sense of security now that the traditional sources of solace (family, career, loving relationships) are less reliable than ever. And online dating offers just such chances for us to have fast and furious sexual relationships in which commitment is a no-no and yet quantity and quality can be positively rather than inversely related.

After a while, Kaufmann has found, those who use online dating sites become disillusioned. "The game can be fun for a while. But all-pervasive cynicism and utilitarianism eventually sicken anyone who has any sense of human decency. When the players become too cold and detached, nothing good can come of it." Everywhere on dating sites, Kaufmann finds people upset by the unsatisfactorily chilly sex dates that they have brokered. He also comes across online addicts who can't move from digital flirting to real dates and others shocked that websites, which they had sought out as refuges from the judgmental cattle-market of real-life interactions, are just as cruel and unforgiving – perhaps more so.

Online dating has also become a terrain for a new – and often upsetting – gender struggle. "Women are demanding their turn at exercising the right to pleasure," says Kaufmann. Men have exercised that right for millennia. But women's exercise of that right, Kaufmann argues, gets exploited by the worst kind of men. "That's because the women who want an evening of sex don't want a man who is too gentle and polite. The want a 'real man', a male who asserts himself and even what they call 'bad boys'. So the gentle guys, who believed themselves to have responded to the demands of women, don't understand why they are rejected. But frequently, after this sequence, these women are quickly disappointed. After a period of saturation, they come to think: 'All these bastards!'"

The disappointing experience of online dating, Kaufmann argues, is partly explained because we want conflicting things from it: love and sex, freedom and commitment, guilt-free sex without emotional entanglements and a tender cuddle. Worse, the things we want change as we experience them: we wanted the pleasures of sex but realised that wasn't enough.

Maybe, he suggests, we could remove the conflicts and human love could evolve to a new level. "If casual sex is to be a game, it has to be based on new rules that make at least some allowance for love. Or if 'love' sounds too off-putting, for a little affection, for a little attentiveness to our partners, given they are human beings and not just sex objects."

This is the new philosopher's stone – an alchemical mingling of two opposites, sex and love. "If that could be done, the micro-adventure of online dating could mean something very different: it could be a way of escaping ordinary life, of enjoying an idyll for two that takes us far away from the world in which we usually live."

Kaufman's utopia, then, involves a new concept he calls tentatively LoveSex (which sounds like an old Prince album, but let's not hold that against him). Kaufmann suggests that we have to reverse out of the cul de sac of sex for sex's sake and recombine it with love once more to make our experiences less chilly but also less clouded by romantic illusions. "We have to discover ways of loving on a strictly temporary basis."

Or, more likely, realise that we can never have it all. We are doomed, perhaps, to be unsatisfied creatures, whose desires are fulfilled only momentarily before we go on the hunt for new objects to scratch new itches. Which suggests that online dating sites will be filling us with hopes – and disappointments – for a good while yet.