An unfamiliar face catches your eye as you scroll down your timeline – someone you may know, according to Facebook.

Yet, on closer inspection you have no mutual acquaintances and there are no obvious links as to why this person has been suggested as a ‘friend’.

Exactly why did Facebook recommend this person to you? What information did it use to do so? And how can you limit the information available to it, should you so wish?

This week we are publishing a series of articles throwing a spotlight on the social media giant’s use of data.

We began with how it targets you via tagging and location then looked at the accessing of phone contacts and email accounts and its analysis of your search history.

Today we focus on  how it uses data from external apps.

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Tinder users have reported their alarm at spotting potential dates they have swiped on the dating site pop up on their Facebook page.

External sites you visit on your phone have been found to affect Facebook, whether that’s through merely having both apps on your phone, allowing access to your phone book or by logging into the external site using your Facebook details.

“We’re in a society now where people aren’t quite as privacy conscious as they were 20 years ago when, if someone asked for information, they would ask why they wanted it,” Pamela Hopkinson, director of Social Media Solutions in Barnsley, told BusinessCloud.

Carmen Lascu, a social media expert and blogger based in Southampton, said: “I don’t think people are aware that this goes on because when you use social media to log into a different website you don’t read what you’re allowing access to.”

WhatsApp and Instagram, both owned by Facebook, also have a bearing on this. Connect with someone on either of the apps and they then become suggestions on Facebook’s ‘people you may know’, according to cyber crime expert Gary Hibberd, managing director of The Agenci, which has headquarters in London and a base in Wakefield.

Gary Hibberd

“If you ask Facebook they say they don’t use information from apps, but I’ve heard of times where people have logged into apps using their Facebook details and then this happens,” he says.

“You’re telling Facebook you’re using these systems and by accepting that they can share contacts details between the two applications.”

In its terms and conditions, Facebook says: “We collect information when you visit or use third-party websites and apps that use Facebook services, such as when they offer our ‘like’ button or Facebook log-in.

"We receive information about you and your activities on and off Facebook from third-party partners, such as information from a partner when we jointly offer services or from an advertiser about your experiences or interactions with them.”

Tomorrow: Precautions you can take