Short answer: Way more than you think. Our own Eli Pariser laid it all out in this eye-opening TED talk, and got a standing ovation for his trouble. His book on the topic, The Filter Bubble: What the Internet is Hiding from You, is out this week.

Here’s what author Bill McKibben has to say about it: “You spend half your life in Internet space, but trust me — you don’t understand how it works.