So here's the thing: every time you "like" something, share something, tag yourself in a photo, or click on an article on Facebook, the site collects data on you and files it away in their folder of YOU. And it's not just your activity on Facebook that they're keeping track of. They also track what device you used to log on, what other app you came from, other sites you've visited, and much more.
All that data helps Facebook paint a detailed picture of who you are and what you like for advertisers. The problem is that we don't know how, exactly, that picture is formed. The algorithms at work are a "black box." We don't know how these algorithms decide whether we're a "trendy mom" or a "frequent traveler." And we don't know how they decide which ads to show us. In short, no one is really accountable.