James Martin / CNET

You don't have to post anything for social networks to learn about you. Your friends are doing all the work already.

A new study from researchers at the University of Vermont and the University of Adelaide found that they could predict a person's posts on social media with 95 percent accuracy -- even if they never had an account to begin with.

The scientists got all the information they needed from a person's friends, using posts from fewer than 10 contacts to build a mirror image of a person not even on the social network.

The study, published Monday in the journal Nature Human Behavior, looked at more than 30.8 million tweets from 13,905 accounts. Using that data, they used machine learning to accurately predict what a person would post based on what their contacts have posted.

So even if you never posted on Facebook or Twitter, it only takes about eight or nine of your friends to build a profile on your likes, interests and personality on social media, the researchers found.

"You alone don't control your privacy on social media platforms," University of Vermont professor Jim Bagrow said in a statement. "Your friends have a say too."

Twitter didn't respond to a request for comment.

The research comes as social networks like Facebook and Twitter have experienced a backlash over privacy concerns. As scandals like Cambridge Analytica made headlines, a movement to leave Facebook arose online.

But the new study suggests that even when you delete your social media accounts, if your friends are still there, tech giants are able to build profiles on you. This is already a concern that privacy advocates have about Facebook, called "shadow profiles."

In April, Facebook CEO Mark Zuckerberg told lawmakers that the social network collected data on nonusers for "security purposes." That includes people's contact list when they use Facebook's mobile app, which the company uses to suggest friend recommendations, it explained.

In response to the study, a Facebook spokeswoman said the company doesn't build profiles on nonusers, even if it's collecting data on them.

"If you aren't a Facebook user, we can't identify you based on this information, or use it to learn who you are," the company said in a statement.

The study shows there's only so much you can control in terms of your own privacy and security online.

In multiple cases over the last year, it was often friends on social networks that led to data leaks. With Facebook's Cambridge Analytica scandal, the This is Your Digital Life app gave the UK firm information on not just the person that clicked on it, but all of that person's friends too.

In Australia, only 53 people actually clicked on the app, but it harvested data on 310,000 friends through that initial contact.

The same happened with Facebook's massive breach affecting 29 million people. It started with 400,000 people within the hackers' network, and quickly spread to friends of friends.

As careful as you are online, the study suggests that you're only as private as your friends have been.

"There's no place to hide in a social network," Lewis Mitchell, the study's co-author and a senior lecturer in applied mathematics at the University of Adelaide, said in a statement.

Cambridge Analytica: Everything you need to know about Facebook's data mining scandal.

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