Facebook allows ad-targeting with shadow profiles

If you thought you have protected your personal information enough from the prying eyes of Facebook advertisers, think again.

When you create an account on Facebook, at some point, you end up providing your mobile number for two-factor authentication, or someone happens to share your email.

While this information is not actively visible on the profile, it is hidden in a layer called 'shadow profile', Mashable reported.

Facebook is allowing advertisers access to this hidden personal information for 'custom audience'. When you provide your data while shopping online, advertisers get access to this data and then show you tailored ads on Facebook, even when you have set your ad preferences otherwise.

(Source)