Short answer: Way more than you think. 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. Your internet is actually being filtered by google and other web sites based on the content you like. Seems like censorship.
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. Eli Pariser’s book is a masterpiece of both investigation and interpretation; he exposes the way we’re sent down particular information tunnels, and he explains how we might once again find ourselves in a broad public square of ideas. This couldn’t be a more interesting book; it casts an illuminating light on so many of our daily encounters.”