I was once having a drink with a Turkish author at the Edinburgh book festival when he announced that he was worried about freedom of expression in the UK. I assumed he'd heard about some crackpot piece of proposed legislation, but he insisted he'd read about a famous British novelist actually being sent to prison. Should he make a public protest? It took me a minute to realise it was a wind-up, and my friend knew perfectly well that Jeffrey Archer hadn't been jailed for anything he'd written.

We don't do things like that in this country, do we? But we do have a worrying tendency to prosecute people for making bad jokes or offensive remarks on social-networking sites.

On Friday, good sense finally prevailed in the case of Paul Chambers, who has been trying to clear his name for more than two years after being found guilty of sending a menacing message on Twitter. Chambers was fined £1,000 for what he admits was a "silly joke" and it has taken three court hearings to overturn his conviction.

I can't see how anyone could read the Chambers tweet as anything other than an outburst of frustration. He was infuriated when his local airport was closed due to snow and tweeted: "Crap! Robin Hood airport is closed. You've got a week and a bit to get your shit together otherwise I'm blowing the airport sky high!!"

It would be unusual for a genuine terrorist to give quite so much notice, or to use his real name, so I can imagine his shock when four cops visited his office in Doncaster and arrested him. They turned up a week after he posted the message, which was hardly a swift response if Chambers really posed a threat to national security.

The key point in this case is intention. Many people use Twitter in the same way as they talk to friends in a pub or bar, exaggerating for effect. When someone exclaims "I could have killed him!", the audience doesn't think for one moment that they mean it. Messages on social-networking sites have more in common with speech than books or articles, which are the product of time and effort. Alarmingly, the Crown Prosecution Service seems to have trouble making this distinction, defending the decision to prosecute Chambers even after his conviction was quashed at the High Court.

I wish people wouldn't rush on to sites and abuse strangers, politicians and random celebrities. I loathe the misogyny and racism on Twitter, and I have no sympathy with people who break the law by naming rape victims.

But if the 2003 Communications Act were to be strictly applied, the courts would be clogged up with people who've posted material that's "grossly offensive" or "of an indecent, obscene or menacing character".

Clearly the law is struggling with developments in technology, which give wide circulation to what amounts to no more than an emotional spasm. But using it in such trivial cases is a serious threat to free expression.

Joan Smith is Political Blonde: www.politicalblonde.com; twitter.com/@polblonde