Some people might think that online privacy（隐私）is a ,well, private matter. If you don’t want your information getting out online, don’t put it on social media. Simple, right?

But keeping your information private isn’t just about your own choices. It’s about your friends’ choices, too. Results from a study show that, with the development of all kinds of social media,people may need to stop and think about just how much they control their personal information, and where the boundaries of their privacy are.

When someone joins a social network, the first order of business is, of course, to find friends. To help the users to find their friends, many apps offer to import contact list(导入联系人清单）from someone’s phone or e-mail or Facebook, to find matches with people already in the network. “sharing those contact lists seems harmless,” says David Garcia, a computational social scientist at the Complexity Science Hub Vienna in Austria. “People giving contact lists, they’re not doing anything wrong.” he says. “ You are their friend. You give them the e-mail address and phone number.” most of the time, you probably want to stay in touch with the person, possibly even via the social media site.

But the social network then has that information—whether or not the owner of it wants it to be shared.

Social platforms’ ability to collect information into what are called shadow profiles first came to light with a Facebook bug in 2013. The bug accidentally shared the e-mail addresses and phone numbers of some 6 billion users with all of their friends, even when the information was public.

Facebook immediately corrected the mistakes. But later, some users noticed that the phone numbers on their Facebook profiles had still been filled in—even though they had not given Facebook their digits. Instead, Facebook had collected the numbers from the contact lists innocently provided by their friends, and filled in the missing information for them. A shadow profile had become reality.

It’s no surprise that a social platform could take names, e-mail addresses and phone numbers and match them up with other people on the same platform. But Garcia wondered if these shadow profiles could be extended to people not on the social platform at all.

He turned to a social network called Friendster which was closed up in 2015. Friendster was launched in 2002. In 2008, the social site attracted more than 115 million users. But by 2009 people began to jump ship for other sites, and finally Friendster closed for good. However, by using the Internet Archive-- a nonprofit library, Garcia still found lots of users’ information about Friendster.

“ You are not in full control of your privacy,” he concludes. If your friend is on a social platform, so are you. And you don’t have a choice in the matter. Garcia published his findings August 4 in Science Advance.