This Valentine's Day, as usual, I received several heartfelt anonymous messages. "You're not funny, you cock", "Why are you such a smug shit?", "Just seen you on a repeat of Mock the Week, I wish you would die". That sort of thing.

But then I get that every day – all comedians do (apart from the funny non-smug ones who are already dead). In fact, everybody does; that's one of the joys of the internet age. On 14 February everyone used to look forward to the possibility that someone would share their passionate feelings incognito, and now it happens all the time. As soon as you have a Facebook wall, a Twitter feed, or simply a name that someone can type, Anonymous Missives Inc is open for business. And it's not only people who are the targets of strangers' ardour – restaurants, bars, hotels, books, movies and DVDs are all the objects of feelings so strong that those holding them are embarrassed to reveal their identity.

I'm sure embarrassment is what it is. Like love, hate is something that makes us go red in the face. It's safer expressed covertly lest it be rejected. If the local cafe knew it was you who found the service unfriendly or the muffins over-priced, it would make you feel vulnerable. This way, you get to call the manageress a wart-faced crone without it getting personal. Anonymity, like a secret ballot, is a guarantee of sincerity.

There was certainly nothing insincere about the 30 negative reviews of The Good Life restaurant in Shrewsbury that were posted online last autumn. They came from the heart. In fact, they came from the same heart: all 30 were written, under different names, by Ms Helen Griffiths, a marketing manager from Salford. But she wasn't managing the marketing for The Good Life – this wasn't an elaborate exercise in reverse psychology. Ostensible offence at "cold and unattentive" staff and "hairs in my quiche" hid Ms Griffiths's real dislike: the vegetarian restaurant's owner, Joanna Langfield. Griffiths was angrier than even tofu can make you, because Langfield is the ex-partner of Griffiths's husband and, last August, became involved in some legal dispute with him.

The online review dispute, in contrast, was deemed illegal. Ms Griffiths, after being given a police caution for harassment, had to publish an apology for the aspersions she'd cast, carefully picking them out of the house hummus and admitting that she'd "never actually visited or eaten at the restaurant". This was the end of a long battle for Joanna Langfield to restore The Good Life's good name in the face of a hate barrage that had caused a 25% slump in the restaurant's profits.

One can readily see Langfield's problem. When a restaurant owner approaches a website to ask for some negative reviews to be removed, saying they're biased, the claim is going to be viewed with scepticism – in the unlikely event that the website has any staff to view it at all. Online reviews, either anonymous or with no verifiable name, customarily go up unchallenged. We assume that the wisdom of crowds will ensure that a fair impression is given overall – that the uncensored self-expression of hundreds of millions will tend towards the truth. Half the time it just regresses to the mean.

And the rest of the time it goes the other way: over-effusive, hysterical praise. So often you'll read a review that couldn't be bettered if the hotelier, restaurateur, musician, bar owner or author had written it themselves. In the notorious case of the description of Orlando Figes's The Whisperers – Private Life in Stalin's Russia on Amazon as "Beautifully written ... leaves the reader awed, humbled yet uplifted ... a gift to us all," it's because he had. But he was even-handed enough to cast his eye over rival works of Russian history, anonymously describing Molotov's Magic Lantern by Rachel Polonsky as "the sort of book that makes you wonder why it was ever published" and Robert Service's history of communism as "an awful book"; and, while sucking on the sourest grapes of all, to write of Kate Summerscale's The Suspicions of Mr Whicher, which beat The Whisperers to the Samuel Johnson prize: "Oh dear, what on earth were the judges thinking when they gave this book the Samuel Johnson prize?"

Figes was unmasked in 2010 and apologised unreservedly for having been caught. But let's imagine for a moment that Figes isn't just a foolish man whose sense of proportion and decency got lost in a research trip to the interminable steppe and give him the benefit of that imagined doubt: perhaps he was trying to teach the internet a valuable historiographical lesson about the limited value of unattributed sources. If you don't know who's written something, you can't know why it was written and so you can't trust it. It might genuinely be a fan of Russian history rightly panning some sloppy research, or a quiche expert correctly informing potential customers that, if there's human hair in it, it isn't vegetarian any more. But, if so, why won't they give their names? If they remain anonymous, there's a decent chance it's an envious historian or the wife of the owner's ex.

When you read a bit of graffiti that says something like "Blair is a liar", you don't take it as fact. You may, independently, have concluded that it is fact. But you don't think that the graffiti has provided that information. It is merely evidence that someone, when in possession of a spray can, wished to assert their belief in the millionaire former premier's mendacity. It is unsubstantiated, anonymous opinion. We understand that instinctively. We need to start routinely applying those instincts to the web.

Some argue that anonymous online commenting should be restricted, that websites shouldn't allow it – they should make you put your name to your words. But that would lead to annoying cries of "Censorship!" and would inhibit the web traffic by which news agencies hope to increase their imperceptible online advertising revenues to a noticeable pittance.

Instead we should merely heed Figes's warning. If you read a review, an opinion, a description or a fact and you don't know who wrote it then it's no more reliable than if it were sprayed on a railway bridge. We should always assume the worst so that all those who wish to convince – whether vegetarian gastronomes or lovelorn suitors – have an incentive to identify themselves.