It's official. British spies no longer wears cloaks and carry daggers. They wear Speedo swimming trunks and Santa hats, and have friends who are actors and Holocaust deniers.

It's official. British spies no longer wears cloaks and carry daggers. They wear Speedo swimming trunks and Santa hats, and have friends who are actors and Holocaust deniers.

The new head of MI6's cover has been well and truly blown. Thanks to his wife's Facebook habit, the world now also knows details of spy-turned-diplomat Sir John Sawers' three children, the flat he uses in London, and information about his parents. Or at least it would have known, had the entry on the popular social networking site not been hastily wiped by a clearly spooked Foreign Office as soon as it was tipped off about the security blunder.

Such details could have been used against us by terrorists and hostile foreign powers, it argued, and might have opened up the Sawers family to the risk of blackmail: Sir John has been closely involved in policymaking on Iran, including Tehran's suspected nuclear weapons programme, Afghanistan and the Balkans. He also worked on Northern Ireland and the implementation of the Good Friday agreement.

Now there are calls for his suitability as head of the British Secret Intelligence Service, in charge of Britain's spy operations abroad, to be reconsidered.

Meanwhile, the entire Sawers family is no doubt in hiding.

The debacle has given rise to questions about how MI6 could have failed, in the age of the internet, to ensure their new man's security. But just how anonymous can any 21st century spy expect to be? After all, we already knew what he looked like, that he'd been given the faintly ridiculous name of "C", and that he'd be writing memos in green ink, the agency's traditional tribute to its first chief Captain Mansfield Cumming, who signed himself "C" in green ink on official documents.

Sir John's professional and personal details have been on Wikipedia, the internet encyclopedia, for months. It wouldn't have been long before Tweeters were following his every move in the strategic battle against international terrorism, weapons of mass destruction and regional conflict. The internet has the uncanny ability to track and record the tiniest detail and there are doubts over whether it is actually possible to erase all unwanted information. Employers can use social networking sites to check up on what type of person a potential employee is and who their friends are.

Secrecy is every diplomat's modus operandi. The quest for virtual invisibility, on the other hand, is mission impossible. But the two are not necessarily irreconcilable.

Ever since Stella Rimington's appointment as head of MI5 was announced in 1992, and she became not only the first to have her name publicised, but to openly pose for cameras, posting information about people running secret services has been fair game. Rimington has gone on to write several books about the agency and is a vocal defender of its openness and transparency.

Her successor, Eliza Manningham- Buller, has revealed - unwittingly or otherwise - the hidden side of the agency's vital work, which requires superhuman levels of willpower, tact, knowing when to shut up and a distinct lack of desire to show off. Manningham- Buller was only one of five people in the world to know that the deputy head of the KGB at the Soviet Embassy in London, Oleg Gordievsky, was a double agent.

He said that it was her ability to keep a secret that saved his life.

No doubt both would have something to say on the debate about whether regulation of the internet is desirable or feasible.

Social networking sites have their undoubted advantages.

They have the potential to help keep individuals in touch with the diaspora of friends and family members and for networking in business. But they cannot differentiate between the friends we want to keep close and the enemies we need to keep closer.

The danger is that we don't notice when they invite strangers in from the cold.