Facebook doesn’t have a great reputation when it comes to keeping your information private. When the company bought the messaging app WhatsApp in 2014, the company promised there wouldn’t be any sharing of private info like phone numbers and profile data between the two; but this year, Facebook started sharing and storing those WhatsApp phone numbers anyway—prompting Germany to issue a regulatory challenge to the practice.

Even seemingly simple practices like allowing users to choose who they share certain profile updates with gets complicated when the company is constantly tweaking its settings, often in the service of making things more and more public. Facebook has, at times, used users’ location data to offer suggestions, though the company quickly said that that was a temporary test feature and no longer in use when the practice was reported in the media.  

Furthermore, most of us don’t know what information we’re giving up when we agree to those long privacy agreements. Thanks to cookies, the social media giant can essentially follow you anywhere you go on the web, whether it’s on the Facebook site or not. So just what does Facebook know about you, and what is it telling other companies about you?

The watchdog reporters at ProPublica want to help you find the answers. They’ve developed a Chrome plugin that allows you to see what information Facebook has about you, from what the company thinks you’ll be interested in to which other companies have your contact information. 

ProPublica’s reporters have spent a year investigating algorithms like those used by Facebook, and here’s how they describe the vast world of how the company creeps on you:

Facebook has a particularly comprehensive set of dossiers on its more than 2 billion members. Every time a Facebook member likes a post, tags a photo, updates their favorite movies in their profile, posts a comment about a politician, or changes their relationship status, Facebook logs it. When they browse the Web, Facebook collects information about pages they visit that contain Facebook sharing buttons. When they use Instagram or WhatsApp on their phone, which are both owned by Facebook, they contribute more data to Facebook’s dossier.

And in case that wasn’t enough, Facebook also buys data about its users’ mortgages, car ownership and shopping habits from some of the biggest commercial data brokers.

Facebook uses all this data to offer marketers a chance to target ads to increasingly specific groups of people. Indeed, we found Facebook offers advertisers more than 1,300 categories for ad targeting — everything from people whose property size is less than .26 acres to households with exactly seven credit cards.

Eek. Isn’t social media fun? According to my results, Facebook knows that Zipcar, The New York Times, and Zillow—whose app I've downloaded on my phone but never created an account with—all have my contact info, because those companies have apparently uploaded a contact list to Facebook with my information in it. It thinks that I will like Farmville. (Please, Facebook, don't make me play Farmville!) I don't tend to "like" that many pages, nor do I put information like where I work on my profile, but the company still knows a startling amount about me, and can predict the movies I want to watch, the music I like to listen to, and how often I travel. 

You can download the Chrome plugin here, and explore the rest of the “black box” series at ProPublica.

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