They don't need to know everything!
With all the information you could ever want right at your fingertips it has become increasingly valuable to large corporations to know what you care about. Your search history, your Amazon wishlist and the people you follow on Twitter are just a few things that can easily be tied together to make a comprehensive profile of your interests, your preferences, your personality.
Among others Google and Facebook are in the business of collecting data about you. How do they do that, you might ask. Well, good question! When you have an account with a social media platform you tend to stay logged in so you do not have to enter your credentials every time you visit the site on your machine. This staying logged in is realised via something called cookies, small pieces of data your browsers stores on your computer. Now, when you visit a website that includes, say, a Facebook Like Button or a Google +1 Link, you looking at this website can be directly traced back to your profile with the social media site and hence to you through these cookies.
This is a technically very clever way that these companies have come up with in order to make ad revenue and god knows what else. Being aware of this fact already makes it a lot less harmful. But recently, I discovered a series of neat little tools that blocks sites like Facebook or Google from displaying their Like Buttons and such on third.-party websites, which essentially kills this tracking mechanism in its inception.
One of these tools and something that I installed on all my machines is called AdBlock Plus and I think their work really makes a difference in what we can consider as privacy these days. With this tool (or similar software) we can make it a little harder for Big Brother to follow our every move.