For all our cultural kinship, Britain and the United States approach expressive freedom in ways that are often sharply different.

The Brits have no constitutional free-speech guarantee comparable to the First Amendment. Their government muzzles the press even without court approval. Official oversight of news media isn’t unthinkable, as it is here, and right now, in the wake of the phone-hacking scandal, an independent commission last week recommended legislation to subject the press to outside regulation.

Yet the British media are nobody’s lapdogs, and reporters can be brazen in confronting authority and courting official disdain.

I recall a recent morning when the anchor for that most establishment of media, BBC radio, interviewing the UK foreign minister about an uprising abroad, said something very much like: “Look here, when are you going to stop sitting on your hands while this man massacres his own people?”— a rude, impertinent and utterly perfect question that would be unimaginable in the often fawning world of high-level, access-driven, U.S. journalism.

Still, it’s fair to say the British are more inclined than we to treat their media as useful and frisky but often foolish and fallible, and deserving, like any powerful institution, of careful watching. Hence, the rise of a judicial system that’s reputedly the best anywhere if you want to sue the media, with courts in England and Wales acclaimed as the world’s friendliest to victims of defamation.

So to the subject of libel via Twitter, and the current flap in London involving Alistair McAlpine, an ex-Conservative Party official who was linked falsely to child sex abuse in a scandalously bad BBC television report last month. The screwup already cost the BBC’s director general his job, and prompted hasty payouts from both the BBC and its rival ITV News.

Now the BBC report did not in fact identify Lord McAlpine by name. What it did was provide clues, and a number of civilians on the Internet messaging network Twitter pursued them and drew the correct — which is to say, incorrect — conclusion about his identity, and before you knew it the hapless lord was being widely tweeted as a sexual miscreant.

McAlpine, in short, was slimed via Twitter. Now, he is going after his tormentors. He has identified 1,000 who identified him in tweets, and another 9,000 who retweeted — meaning, passed along — those messages. From them he wants apologies.

Beyond that, he is pursuing 20 “high-profile Tweeters”— those with 500 or more followers, notably a TV comedian, a newspaper columnist, and Sally Bercow, media celebrity and wife of the speaker of the House of Commons, who had some 56,000 Twitter followers. From them he wants money.

Now, McAlpine’s isn’t the first case of Twitter libel. Courtney Love, the rock diva, paid out $430,000 for disparaging a fashion designer in 2011; a famous New Zealand cricketer, Chris Cairns, was awarded 90,000 pounds in March 2012 over tweets from a Pakistani sport official accusing him of match-fixing; and there’ve been others.

But those were straightforward defamations; the libel just happens to have been committed online. The McAlpine case is different. Here, the wrong in question was done not by the originators of the libel, but by civilians who passed along what they’d heard.

Of course, that reflects the brilliance of Twitter, the global backyard fence, a networking phenom that enables communities, vast and small, to coalesce and dissipate around common interests — and fleeting gossip. But what about when that gossip is false and damaging, and it’s being shared not with your neighbors but with thousands of communicants, who share it with still others?

One-on-one talk, even mean–spirited, deserves protection. It’s just private chatter, nobody’s business but our own. But what about when thousands of people listen in? Don’t we leave the realm of conversation and enter the world of publication?

I don’t know when tweeting becomes publishing, but assuredly, at some point, the distinction disappears. News proprietors who failed to take the proper steps to make sure what they published was true have paid damages over libels that reached far fewer than Sally Bercow’s 56,000 Twitter followers.

Even though U.S. law is more forgiving of Internet-borne injuries, social media users should be mindful that their rumoring and speculation may very well leap the bounds of what the Economist felicitously called Internet “birdsong,” and might instead have vast and harmful reach.

If Twitter is, as its advocates say, a powerful and even transformative technology, those who populate and animate the network must accept that with power comes responsibility, and add caution, compassion and accuracy to the principles that guide their usage.