Digital abstainers who refuse to sign up to social media sites may believe they are protecting their privacy, but a new study shows they are still highly visible through the accounts of friends and family.

Around one in five people who use the internet in Britain do not have a Facebook account.

But researchers found it was still possible to predict their future activities, with 95 per cent accuracy, simply by studying the profiles and posts of eight or nine close contacts.

Experts said it was like overhearing a mobile phone conversation on a bus, where you can infer details about the person on the other end of the line, but with a thousand conversations rather than just one.

The team of scientists, from the University of Vermont and the University of Adelaide also gathered more than thirty million public posts on Twitter from 13,905 users to see if they could forecast what someone would tweet.

Again the information from friends and family made it possible to predict a person’s later tweets as accurately as if they had studied that person’s own Twitter feed.

The researchers said without realising it, social media users were leaving ‘digital traces of interactions’ like breadcrumbs which created ‘shadow accounts’ for people who thought they were exempt.