How to find out what Facebook knows about you

With more than 2 billion monthly active users, Facebook can keep tabs on nearly a third of the world's population. Whether you visit the social network daily (as 1.32 billion people do) or only log on to RSVP to events, you should be aware of how much of your personal data you're giving to the site, and the company behind it.

Facebook primarily employs your information to serve you more relevant targeted advertising. While some see this as uncomfortably intrusive, others accept the ads as the price they pay for the network's free services and tools. Whatever you think about the ethics of this data collection, you should know just what the company is learning about you—and how you can control the flow of information. While this guide can help, there's only so much you can do to protect your privacy—if you really want to stop Facebook from mining your data, your best bet is to delete your account.

When you first set up your Facebook account, you entered basic background information, including your name and email address. The site also prompted you to fill out your location, work and education history, and friends and relatives currently on Facebook. But by observing your behavior on its network, this tech company has gathered a lot more information about you and your habits.

One quick way of checking exactly what details Facebook knows—and limiting who else can access them—is via the ad preferences page. (If you'd prefer to get there without a link, log into the site and click the arrow on the far right of the navigation bar across the top of the screen; from the drop-down menu, select Settings then Ads.) Here, you can see the information you've given to the social network, and determine which of these details Facebook should be allowed to share with advertisers.

For example, click Your information to remind yourself whether you've shared your relationship status and employer with Facebook. Hit Your interests to see the topics the company thinks you enjoy, determined by analyzing the pages you've liked and your online (even non-Facebook) browsing activity. To stop Facebook from showing you ads based on these details, turn the toggle switches to off or click the x icon on the topic of interest.

This doesn't require you to eliminate helpful details from your profile—you can share, say, your relationship status but block ads that target you because of it. If your uncomfortable giving this information to the social network, then you can delete it. Facebook also offers a fuller explanation of ad preferences.

Even if Facebook isn't selling your information to advertisers, it could still be collecting it. In addition to making ads more relevant, the company can put your data—everything from the make and model of your phone toyour most frequently used apps—to work fixing bugs and improving the social network. you can delete some of this data completely by going to your Facebook profile page (click your name on the toolbar at the top of the Facebook web interface, then click Edit Profile). You can't erase everything, but you can unlike pages, delete education and employment details, and more.

You'll never find every last detail that Facebook knows—or thinks it knows—about you: Its secret algorithms make some educated assumptions about who you are based on your profile and your online activity. However, it's not a completely closed black box. In addition to checking your recent activity in the Activity Log (to access it, click View Activity Log on your profile), some third-party services will help you hunt for your information.

The Chrome extension Data Selfie can monitor your Facebook activity, giving you a few more ideas about the sorts of ways Facebook might track and interpret your data. However, the extension's conclusions are mostly theoretical, rather than accurate measures of what Facebook knows about you.

For an even more comprehensive analysis of your account, visit Stalkscan. When you enter your Facebook profile URL at the top of the page, the site will use your profile to glean details that only Facebook knows about you, from the hotels and bars you've checked into to the videos where you've been tagged to the music and movies you've liked.

You can run similar searches from the Intel Techniques Search Tool website. At that site, click on Facebook on the left side of the screen and enter your profile URL in the Facebook User Number box to retrieve your Facebook ID number. Then you can look up links such as the photos you've liked, the videos you're in, the (public) groups you've joined, and much more.

Unfortunately, websites like these only scratch the surface of Facebook's data gathering. For instance, the social network carefully curates your News Feed based on your past interactions with posts and people—but won't reveal the details of the algorithm that helps it do so. In addition, Facebook makes assumptions about its users to it can categorize people into groups that advertisers can target. It doesn't really matter if these assumptions are always right, as long as they make a more efficient advertising platform overall. Last year, the Washington Post published a report on 98 different data points Facebook associates with your identity. These include data pulled from other companies and services—like what year you bought your car and what type of credit card you carry.

Beyond the information you give up in your profile and the pattern of your clicks (from likes to photo comments), Facebook collects two other big pieces of data: Your location (determined via your smartphone) and your activity elsewhere on the internet.

Letting Facebook's phone app know where you has some upsides: It enables you to check into places, search for interesting spots nearby, and even find your friends more easily. It also tells Facebook where you tend to hang out, allowing the service to be more precise about the ads it shows you.

If this makes you uncomfortable, you can turn off its ability to keep tabs on your whereabouts. On Android phones, open Settings, tap Apps & notifications, App permissions, Location, and remove Facebook. On an iPhone, open Settings, tap Privacy and Location Services and remove Facebook. Even with these precautions, Facebook still can keep tabs on you—for example, it will take note when your friends tag you and also tag specific places.

Similarly, there's a simple reason why Facebook tracks you around the internet: better advertising. Facebook can receive notifications when you spend time on specific webpages. In addition, the marketing platforms and sites to which Facebook sends your information can also send the social network their own carefully gleaned data about you (this explanation has more information about the practice). What's in this data? You can't know exactly, beyond checking the Ad Preferences page we've already mentioned.

To prevent Facebook from following you around the web, go to Ad Preferences, open Ad settings, and change the top option ("Ads based on your use of websites and apps") to Off. You can also visit the Digital Advertising Alliance and specifically opt out of numerous cross-site tracking programs, including the one run by Facebook.

These days, Facebook takes more care to explain what type of user data it collects, but you still can't reclaim all of your information—that's something you sacrifice when you agree to sign up for the service. What you can do is be more aware of the types of information you reveal as you fill out your profile, react to your News Feed, and browse the web.