Facebook has a lot of information on you. Even if you don't think you give away your political leanings on Facebook (e.g. you make a point of not sharing or liking political posts, for fear of upsetting a relative with opposing views), it can still make guesses about you based on the likes and preferences of other users who do express those beliefs.
If you like something from a comedian, for instance, you could very easily give away your political beliefs. Or if you like a page, let's say Hummus Memes, for example.
This won't give away your preferences directly. However, Facebook may know that the majority of hummus meme fans are alt-right sociopaths from their likes (purely for the sake of the example).
If you happen to like a few other pages (or regularly like posts from pages) that have a big alt-right sociopath following, Facebook can make an assumption that you too are an alt-right sociopath. All from your enjoyment of hummus memes and a few other clicks on the "Like" button.
It then labels you as this for advertising purposes. Advertisers may want to target liberals or conservatives specifically, and Facebook allows them to do so. This could be for reasons such as they want to target the young and trendy liberal crowd, or they know older conservative dads tend to enjoy whiskey. It can also be used by political campaigns, who want to target a specific demographic, e.g. a campaign to get liberals out in key states on voting day.
Rather than target everyone, they're better off just targeting people who might listen to their message, which Facebook can help them do with astonishing accuracy. How effective this is, and whether it helps to alter the outcomes of elections, isn't that clear.
You can see what Facebook has guessed about you through the following steps.
Head to this URL: https://www.facebook.com/ads/preferences
Once you're there, click on the "Your Information" section. Then click "Your Categories" where you should see a category called "US Politics", which gives your supposed political leaning in brackets.
Here it will have labeled you as anything from very liberal to moderate to very conservative. How accurate was it for you?
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