Another Way FACEBOOK Spies on You

      We're all used to seeing "Log in with Facebook" or "Log in with Google" at sites around the Internet, such as CNN.com, the Guardian newspaper, Spotify, and tens of thousands of other online retailers, apps and games. Facebook gets 62% of all social log-ins across the tens of thousands of sites and Google is used 24% of the time. 
      It should be obvious that every time this is done, one is surrendering personal information to FACEBOOK's or Google's dossier on the user. Logging in to a website using a service such as Facebook or Google allows the website to make a request for data about you -- your birthday, friends list, email address, employment, colleges attended, photos and information that your friends have posted about you.  This data covers your habits and preferences, your Facebook Likes, products or articles you've liked. and which of your friends already use a particular website or what you do while on the site. 
      This can have serious repercussions. Allowing another account who has weak security practices means if this account is hacked, your data may be compromised and you will never know or be notified.
     People who use Facebook and Google log-in trade away some data privacy.

• Personal data is shared between the social network and the third-party app.
• Don't link a social profile to sensitive info like your Social Security number or financial details. Always use a separate account and password.

https://www.techlicious.com/blog/should-you-use-facebook-or-google-to-log-in-to-other-sites/

A SHORT HISTORY OF FACEBOOK'S PRIVACY GAFFES
https://www.wired.com/story/facebook-a-history-of-mark-zuckerberg-apologizing?mbid=nl_033118_daily_list1_p2&CNDID=48648177