It was to be expected that the BBC would be forced into a legal settlement with Lord McAlpine after Newsnight's failure to carry out basic journalistic checks resulted in his being wrongly identified as a paedophile. What may prove of greater significance, however, is that the peer also intends to pursue those who explicitly linked his name to the story – and that was not the television programme, it was individuals using Twitter.

There were more than 3,000 mentions of the former Conservative Party Treasurer on the social network on the day of the Newsnight broadcast. Lord McAlpine's lawyers have now enlisted a specialist company able to recover even deleted tweets to track down anyone who accused him of child sex abuse. Not only is he well within his rights do so; he may also set a new legal precedent in the process.

Twitter enthusiasts compare its intimacy and spontaneity with a chat over a drink in the pub. But the laws of libel still apply, even if the medium appears to blur the boundary between public and private discourse. Traditionally, the author, editor, publisher and printer can all be sued for libel – raising questions about how far Twitter itself is analogous to the letters pages of a newspaper, say, or to a conduit such as the telephone network. Free speech concerns must surely render the latter a more apt parallel, putting the onus on the individual users, rather than the company itself, to take responsibility for the content on the site. Lord McAlpine is, therefore, right to focus his attention on the tweeters themselves – both those who named him, and those who forwarded on others' slurs.

No distinctions will be made for scale. The most prominent tweeters who have tens of thousands of followers – the Speaker's wife Sally Bercow, say, or the columnist George Monbiot – can expect to receive lawyers' letters. But so can those whose tweets reach just a few.

Legal precedent is steadily catching up with the complications of the internet. While Newsnight did not name Lord McAlpine – referring only to a senior Conservative from the Thatcher era – it did provide sufficient information to allow others to identify him on the internet and to make libellous inferences as a result. So-called "jigsaw identification" has been prosecuted before, when the footballer Ashley Cole successfully sued the News of the World, in 2006, for publishing an obscured photograph of him which could be used to find the original on the internet.

There has also already been one successful Twitter libel case when former New Zealand cricket captain Chris Cairns was awarded £90,000 damages in March over a defamatory accusation of match-fixing. And earlier this month nine people who illegally named a rape victim on Twitter and Facebook were made to pay her compensation. Slowly but surely, awareness is growing of the legal risks of careless or malicious use of social media – and Lord McAlpine's action is another step forward.

Such a development can only be construed as progress. The mass communication enabled by social media sites has many benefits. But questions of where responsibility for potentially illegal content may lie remain unanswered. What is clear is that, without consequences, Twitter and its ilk leave too much space for cruelty, injustice and a dangerous mob mentality. The wrongly accused Lord McAlpine is a case in point. As the peer movingly described yesterday, trial by Twitter is both terrifying and leaves a legacy that "cannot be repaired". It can only be hoped that his lawyers' efforts will incline tweeters to weigh their electronic words more carefully in the future.